Hiya! Welcome to GH!
Your choice of the AT101W is a very good one for a first keyboard. It's an extremely well-rounded keyboard that's easy to get and very cheap while being sturdier than most newly-made keyboards. And it looks kickass.
For the Dell AT101W (FCC ID GYUM90SK), there shouldn't be any difference really. Malaysia or Mexico, they use the same case and appear to use the same switches, Alps SKCM Black/Late. FCC ID GYUM90SK I've had a whole bunch of, several of which I disassembled, and they were all the same. All basic, good keyboards.
Other FCC IDs were very different though. Try not to think of it in terms of place of manufacture too much with this board, but in terms of WHEN it was made. Older versions, before the GYUM90SK, had one or two slightly more high-grade specs here and there, such as higher-quality lettering on the keycaps, or going back even further, more upmarket switches. As far as I know all GYUM90SKs are the same, though.