I've got an older Bernina 006 w/o differential feed. I got an amazing deal on it on ebay and love using it! It's a seriously hardy machine/goes through anything and is very solidly built. However, since I've started sewing w/ knits/velours a lot more I've missed having the differential feed. It really makes a difference in how knits serge up. I just got a Pfaff 4872 that I also

. It's got differential feed (a must, IMO) and coverts to do the coverstitch. I'm still learning my way around it, but so far it feels really great!
I'd reccomend taking a load of fabrics into your local sewing machine shops and playing around w/ lots of different ones. I'd also go over to
www.patternreview.com and read reviews on some of the different ones. Also, and this sounds strange, but I'd really see if a machine "feels" right to you before you buy it. Both my Bernina and Pfaff sergers "fit" me. I feel like we get along well and I feel comfortable using both. Now, my new sewing machine on the other hand. We don't "fit" as well. Kind of like 2 people who keep trying to be friends, but it's just not meant to be. I think I'm going to take her back in and trade for something else. I wish I'd spent more time sewing on the machine I bought and really got a feel for her first, but I didn't feel like it was necessary at the time. Now, i'll never buy a machine w/out spending more time on it at the shop regardless of whether or not I feel like the salespeople want me to!! :LOL