I'm not exactly sure about the differences in the gears, but the 554 has rear hydraulic remotes and the 354 doesn't. They are necessary if you want to add hydraulic operated equipment on the rear. Also, the 3 point hitch on the 554 picks up twice the weight than the 354 and the 354 is only a cat 1 and the 554 is a cat II (I believe mine is a cat III) which is a lot more beefy. The bottom line is: you can make a bigger tractor do smaller jobs, but you can't make a small tractor do bigger jobs.
Corsair