Okay, here is my two cents based on 30 years of messing about with tractors. I'm a bit biased - bought a dk55c two years back....
I have some tractors with hydostatic drive and some with shuttle shift - like the dk55 and some with older standards. If you do a boat load of loader work, the hydro is real nice. If you need the power at the pto or just overall, the go with a shuttle shift, they really are quite nice today.
Now for the size. I say buy the heaviest thing you can afford. Tractors today have plenty of power for what most folks do, but in some ways, way too much power for the frame. If you start lifting things with a loader, and you live around hilly terrain, you could flip the ole tractor quick - be careful and carry loads as low as you can. I've got the dk55 tires loaded and carry heavy implements on the back when messing with my grapple bucket.
Just remember, if you are the kind of guy that will tackle just about anything, get a big brute, and you'll do fine. If you are more careful and think before you tear into something - wish I did! - then the smaller hydros are pretty sweet!
Oh, did we mention money.... that of course is never a factor... right?