The alluminum trailers have large I beams running the full length of the trailer and they are just as strong as the boxed steel with less weight. The biggest issue of roller -v- bunk up in the Northeast is tide height which makes for steep ramps.
The bunk trailers are more float on / drive on and the rollers are technically winch on. Many float bunk trailers don't even have a winch - see this tandem trailer for a 22 footer :
Now I used to drive on my roller trailer but a lot of NE ramps frown on that today and some will fine ya. Down south the bunks are popular becuase they have nice easy & shallow ramps and the trilers are easier. Up here, where you get the bigger tides & steeper ramps it's a lot harder to "float" your boat onto the trailer because of the ramp angle. Which is why roller trailers are big up here.
Bunk trailers also do a better job of distributing the weight accross the bunks leaving more fiberglass of the hull to support less weight of the boat whereas with rollers, the weight of the hull is distributed across the small sections of glass making contact with the rollers. This is part of why a lot of boat builders want a gazillion rollers if you use a roller trailer. I've even heard of some Internet horror stories of boat builders not honnoring warranty on hulls trasnported on roller trailers with large gaps between the rollers. I don't know if that's Internet crap or not...