these are different generations. 75GXP then 60GXP then 120GXP
the newer drives are more dense, that is, have more gigs per platter which increases speeds some. The 75GXP were all the rage when they were out. But ppl had problems with them. Then the 60GXPs supposedly fixed problems, but some drives (much fewer than the 75s) also crapped out. the 120GXP is still fairly new. PPL dont go out and buy drives all the time so i have not heard much of the 120GXP save the reviews stating they are the best of the best. Of course we could always get into the WD 120GB drive that has 8 MB of cache.
Frankly I like IBM drives, so get the 60GXP series or the 120GXP series.
And yes, the numbers of the generation do refer to the highest capacity drive offered (in gigabytes).