It will first depend on your budget. Once that is established then you get to decide which features you like.
Do you want:
- a new machine that does the wave edging or not
- the cover stitch and chain stitch
- self threading
- automatic tension setting
- new vs. used machine
I have an older Pfaff and haven't found threading difficult at all. I made my own color coded guide in my own language when I first got the machine. I absolutely love it. Do I wish it had more features like the wave edging? Yes. Would I use those features enough to make that investment worthwhile? Probably not.
Try out different brands and models and try to negotiate.