larger diameter rollers offer less resistance than
a small diameter roller, they are also a little easier
to learn on (although you are "higher" off the ground which can be disconcerting to say the least).
I have both Alu and PVC rollers, the Alu are a bit quieter
and feel smoother. I once asked same question about differences and got response "Aluminium rollers dent when you drop something on them" haven't had that problem (yet).
Kreitler are definately the best out there, but are pricey.
I suggest getting something less expensive to make sure
you really like them.
I'm of the opinion that resistance units are not needed, that rollers
are for spin/cadence and balance, not strength, just my .02 worth tho.
Do a search on Rollers here, there are alot of posts/threads concerning rollers.
Marty