If I could afford it (and if I had grandchildren!!!) I would get a self-threading serger that includes the cover stitch (or one self-threading serger that does everything but the cover stitch and another serger that does just the cover stitch). A serger is a machine that tends to last a lifetime, so you really want to get the best if you can afford it.
The self-threading feature simply means the serger becomes *much* more user-friendly; you can sit down and start sewing immediately instead of spending hours fiddling with threading and tension.
The cover stitch is ***really*** nice to have if you are making clothing. Look at the bottom of a t-shirt to see what a cover stitch does. On the right side it looks like two perfectly parallel lines of sewing; on the wrong side is a zigzag-type stitch that covers the raw edge of a knit. This is SO wonderful to have for giving a finished look to clothing! Although you can achieve a similar effect on a sewing machine by using a double needle, it's not nearly as easy or neat that way because the differential feed on the serger compensates for the stretch in knit fabrics.
I have a good Bernina serger that is not self-threading and that does not have the coverstitch. I purchased it about 15 years ago and it still works fine. However, I never used it as much as I expected for making my dd's clothes because it just wasn't that easy to use.