By a total fluke, I have for a short time both the Bernina 830 and the Pfaff CS. Both have their strengths and weaknesses, and it is very hard to say which one is better. Here's some thoughts based on my experience, in no particular order:
The Pfaff handles just about any type of thread that you can throw at it with nary a hitch. Thread nests and breaks are very rare, even with metallics and monofil. The 830 is picky about threads. Sometimes you need to play with the tension, needle type and size, and other settings to get it to behave.
The two adjustment knobs on the 830 are WONDERFUL! Turning a knob a half turn beats sitting and tapping the screen 6 times any day.
I find the interface on the 830 to be more intuitive than the Pfaff, which typically requires that you drill down several levels to get the function you're looking for. The Pfaff has more colors on the screen, but it is slower to load a new design or when you change views. The 830 shows exactly which stitch you're on when embroidering whereas you have to zoom the screen on the Pfaff to get the same detail. The central placement of the 830 screen is much easier for me to use than the side mounted screen on the CS.
The Pfaff rarely needs cleaning and is internally lubricated so you never need to oil it. The Bernina must be oiled regularly and is VERY picky about lint, threads, or other tiny bits of gunk in the works. You really should clean the 830 after about every 4 hours of sewing, more often if you're using cheap or very fuzzy thread or fabric.
The presser feet on the Bernina are sturdier and just have a more solid feel. They hold and move the fabric very well. They are also much more expensive than the feet for the Pfaff. The Pfaff feet control the fabric well, but sometimes feel loose and flimsy.
The feed dogs on the Pfaff are more precise for piecing. The Bernina feed dogs tend to let a 1/4 inch seam wander a bit at the end. Both machines have a dual feed arrangement. The one of the 830 folds way up out of the way when not in use. The Pfaff folds back but not so far, making it a bit easier to catch if you are using bulky fabric. Both help to match top and bottom fabric, but neither is as good as an actual walking foot, which you can get as an accessory. On the 830 you can adjust the dual feed to feed the top and bottom fabrics at different rates. This is helpful if you have really slippery or stretchy fabrics.
Because of the new thread tensioning system the stitch quality on the Pfaff is excellent in both sewing and embroidery modes, though the sewing stitches tend to be bit tight and may pucker lighter fabrics without some adjustment. Embroidery stitches are very well balanced with the back of the design looking almost as good as the front. The built in decorative stitches typically need some adjustment in order to get them regular and lined up.
The sewing stitches on the 830 are also excellent with a bit less pull on the fabric than the Pfaff. The decorative stitches are consistently even and balanced. Embroidery is exceptional when it works and a chore when it does not. Some parts of a design may stitch out beautifully, then you get a run of skipped stitches and birds nests. This is usually because the bobbin area needs cleaning.
In the arena of embroidery quality and consitency, and the ability to easily handle different weights and types of thread, the Pfaff has the edge.
Buttonholes on both machines are good but not exceptional. The number and types of buttonholes are about the same. One nifty feature on the 830 is that you can hold a button up to the screen and turn the adjustment knob to make the buttonhole size match the button diameter and thickness.
The auto threader on the 830 is a joy. You have to make sure you get the thread set correctly, but once you do the machinery does the work for you. It is not as quiet or smooth as the one on the Babylock (different machine, I know). The manual threader on the Pfaff is OK but the little pin that threads the needle tends to get out of whack very easily.
Overall, the Bernina has a more solid, quality feel than the Pfaff and is quieter in both sewing and embroidery modes. Pfaff quality has deteriorated since the company was purchased by Singer/Viking.
Bernina is far superior in their education offerings. They have very clear tutorials on the website for just about every function and project you can think of.
Two final thoughts. First, base a big part of your decision on the dealer. They are the ones who will get you through the initial learning phase, help you master the amazing capabilities of these machines, and keep your machine in good running order. Are they responsive to questions? Do they take time with you, or do you get the feeling that if you're not prepared to buy on the spot, then you're wasting their precious time? Will they give you a loaner machine if yours is out for repair?
What classes do they offer? Ask to sit in on some of the classes so you get a feel for the teachers. Are they patient and knowledgeable? Do they take the time to show you how to problem solve when things don't work out exactly right?
Second, go back and test the dickens out of the machines, but make sure you do the same things on both of them. Take in several different types of fabric, not just the samples the dealer gives you. Sew on thick and thin, wooly and silky, knit and stretch. Sew a straight stitch in different lengths, some zigzag and speciality stitches, and several of the decorative stitcthes. If you do free motion quilting, test that out. Ask what type of thread the manufacturer recommends. Sew with that thread and then some of your own. Sew with metallics, monofil, and 40 wt cotton. Sew with different weights in the bobbin.
Find an embroidery design that incorproates a variety of stitches; satin, outline, running, candlewicking, motif fill, etc. Use the same stabilizer, bobbin, and top threads for both machines, then compare the output quality and how much adjustment you had to do to achieve it. .
Listen to the machines and pay attention to how they feel when you change feet, use the auto threader and cutter, or make adjustments such as moving the needle positions. Do they clunk and rattle, or is the motion smooth and quiet. Play with the interface. Is it intuitive to get to where you want to go? How many movements do you need to do to select or adjust? Are the controls easy to reach and responsive? Make sure to test them all.
Tell the dealer that you will probably be on the machine for an hour or more, and that if they need to take a break to take care of other customers that is perfectly fine. If they don't want to take the time, that tells you a lot about what your future relationship will be like.
If one of these machines was spectacular and the other was terrible, the choice would be easy. As it is, they are both very good machines that have their own strengths and weaknesses. The Bernina is finicky, but solid and very high quality. The Pfaff does not require as much attention, but is not as intuitive or user friendly. Just don't be afraid to spend a lot of time to get to know them both before you decide. You will be putting down big bucks and will most likely be sewing on your choice for many years.
I'd love to keep both machines but that's not financially feasible. I'm currently leaning towards the Bernina because of the dealer, the feel of the machine, the interface, and the output quality. The downside is that it's fairly picky and high-maintenance.
You really can't go wrong with either of these machines, it's just a matter of which one you're more comfortable with. Good luck and happy sewing/quilting/embroidering/etc.
DB