I just got a Pfaff powerquilter 16 and am having a very difficult time turning it off! It's basically the same machine as the Sweet 16 and Tiara and I believe the pricing is the same on them - or at least so said the sales person. In a week I've put on 130,000 stitches and have finished 2 small pieces along with lots of test pieces.
On yahoo groups there is a group for HQ with lots of sit down quilters on there and there is daily activity. Lately it's been discussions on bobbin tension, cleaning the machines, and needles. I've learned lots there as well as on this group.
These groups have been awesome!

I am agree with the other ladies, you better off with the HQ. "I always say you get what you pay for". I think you will be spending double at the end if you go with the other machine.

I have had my Sweet 16 sit down for just over a year and just finished a quilt 66x82 on it and I am a FMQ rookie. You can use long arm templates and rulers 1/4" thick on the Sweet 16 which is awesome. They take a bit of practice managing them, but I get more comfortable with them as I go and have great results. One thing I would recommend to anyone, is to get yourself a couple of extra tables at Walmart or costco to put on each side of the machine table, rather than spend the $200 each for the HQ extensions and make yourself a suspension system for lifting the quilt off the table surface while you quilt. Unbelievable difference in managing the bulk. I feel like I can quilt any size now and I'm not wrestling the quilt. Also would recommend the Q snap frame clamp pieces over the front of the table edge so the quilt doesn't catch on the edge. Here's a pic of my setup. My husband made the frame for me, repurposing the roof rack of our old camper and some 2x4's. I LOVE IT!!!! To give credit where it due, I first saw this idea in Leah Day's website and then several others thereafter. Great if you are a DSM quilter too.