I have an older four-thread Bernina that works fine. However, if I had grandchildren or some other reason to do a lot of serging, I would invest in (1) a serger with air-threading and (2) a separate coverstitch serger. Air-threading would have saved me a lot of frustration in the past, plus now that my eyes are not quite as sharp and my hands not quite as nimble, air-threading would be a wonderful blessing. A coverstitch serger does only one stitch -- hemming -- but would be wonderful for any kind of serged clothing. If you are a creative person, you might want to look at the more expensive sergers that have more harp space and can do many different kinds of decorative stitches, such as wave stitching. I personally wouldn't get one of those because, just as I don't use many of the decorative stitches on my sewing machine, I probably wouldn't use many of the decorative stitches on a serger. Also, the learning curve is steeper for the fancier machines.
I have seen so many posts about the air-threading Baby Lock sergers, that is probably the brand I would get. Here is a link to their many variations:
http://babylock.com/sergers/
I would probably get a 4-thread serger and a coverstitch machine. My interests would be primarily in making children's clothing (if I had any grandkids to sew for!). I have serged a few quilts but do not like using a serger on quilts. The seams are bulkier than I like, and it is much harder to be accurate piecing on a serger than a sewing machine.
Edit: There are quite a few YouTube videos that demonstrate what different sergers can do.