Couple things to think about when buying a trailer. First one you already answered, 90% of the time you'll have a sportsman and a rzr on it. Which is exactly what I do to. Second, how much room do you have to park this thing both at home and where you ride? Third is budget, fourth is what else do you haul (tractors, cars etc.), fifth is tow vehicle.
My 16ft black deck car hauler is great with a sportsman parked sideways in the front and rzr behind it with plenty of room for cargo around them. In that 10% where you are hauling a second sportsman can you put it in the bed of your tow vehicle? Parking was a big issue for me. I got a 16 (overall length is 20 remember tongue length) because my parking area was small and I didn't want to move it around every time I mowed grass. Budget for a couple reasons. Aluminum is very nice but almost triple the cost of a steel in some cases also the longer you go the more expensive. If you are going to be hauling other things make sure its heavy duty and long enough to haul them. No sense in buying a trailer that won't do everything you want, that's the purpose of getting a new trailer anyway right? The tow vehicle is one of the most important. You have to buy a trailer you can move! A 20ft goose neck is going to look really funny with its hitch going through the back window of a Ford escape, not to mention not being able to move it empty let alone loaded. Just be mindful with what you buy and what you can tow.