I prefer the floating deck. Some peolpe like their grass shorter or taller than others, and it seems that the resale value of the floating deck would be retained, so not ever lost. Of course, I only had a Deere/Bunton fixed deck, and had to be unbolted to change height. (unlike the Exmarks that adjust alot easier) but with the HP, no matter what height, the deck-pitch stays the same. As far as hydro or belt drive, just a personal preference.
I had to go with a 48", cause it seemed of more value vs. money, considering the extra foot of cut. I had no gates to get through.
A did pick up some gated yards, not too long after, and the 999.99 price tag of the 21" Metro just seemed a bit much, to say the least (I'm cheap). It seems like with most, a 36" is a must have item, and a good starter mower.
I never had any slipping probs. with the Exmark belts, and the belt tension springs can be adjusted if I did. I just hated the sound of the wet belts, and didn't see that it could be good for them.
(not that it hurts them) I just super-baby mine, and don't like it (or me) to get wet, unless I'm doing the wash/wax thing.