Weight of Trailer ?

I have a small but beefy flat bed trailer with 15" wheels and a 3500 lb axle. Even though it is only 5.5x8, it is about 700 lbs empty and has no side rails. Your biggest concern will be break over angle of your Harley at the ramp transitions and sufficient width for your Can Am to clear the wheel wells. Large wheels will tow better and be easier on the bearings than small wheeled trailers. Depending on CA laws, you may need brakes on the trailer for the amount of weight you plan to tow - My state does not require brakes on trailers tagged under 3000 lbs.

I tow up to 2000 lbs with my VW TDI and can feel the difference between 1000, 1500 or 2000 lbs. Towing the same trailer with a mini truck or larger, I could not feel the difference. I think you should go with the aluminum trailer for corrosion prevention more than weight savings in your case. If you live in an area where you will be pulling it through salted or chemically treated roads, the aluminum one will require less upkeep. My trailer does not have brakes and I can feel the push on the VW and mini truck when decelerating, but not on the bigger vehicles - You may want to look into simple surge brakes to save your FJ brakes.