I have a 2 place 3500 single axle trailer constructed with tube steel and 2x6 treated deck. Loaded with2 quads, quad trailer, gas and gear any decent sized tow vehicle (V8)it pulls very well with the 15" tires. I also have a tandem 18 ft flat deck also constructed with tube steel and 2x6 treated deck. Loaded with 4 quads, quad trailer, gas and gear pulls as well with the same tow vehicle.The biggest disadvantage to the tandem is ability to take it far into the bush or manveuring.

I agree buy a trailer that will suit now and future needs. If you may need a 2 place then buy it now vs later, as mentioned can never have to much space when embarking on a trip. I originally had a single place metal grate trailer, it was great for manveuring but when 2 bikes where needed space was a concern. To me it easier to spend a few extra in the first place then having to trade up!

As far as deck material I saw no advantage to the steel grated trailer, as I wash my bikes off the trailer and then hose or sweep off the deck of the trailer if needed. I seal my treated deck each year with some thompson clear sealant and GTG. My 2 place is 8 years old and my tandem is 4, for a place that sees moisture in the form of rain or snow I am very pleased with the returns of treated wood decks. I touch up the tube steelwhen ever requiredto prevent rust forming from the rocks and salt it sees each year.