Next step up from 3500's is 5200's, which is what I have on my trailer. Although plenty of people do it, I wouldnt plan on regularly towing a fullsize on tandem 3500's because once the trailer weight, vehicle weight, and then all kinds of extra crap are added on you are approaching the load limit of the axles every time you use the trailer, which is obviously harder on the bearings and brakes.

I personaly prefer wood decks, steel decks get hot as a skillet sitting in the sun and I hear they weigh more. Steel is deffinately more expensive too.