Of the two HF mini trailers, I'd get the 90153, not the 42708. Look at the tire differences and the speed ratings. You'll spend your savings trying to upgrade the lesser.
Both are absurdly heavy duty for behind a motorcycle, but they are cheap. Put a box on either and you'll rapidly find yourself closing in on 200 lbs, not 150 lbs. Boxes weigh a lot more than you're given them credit for.
The spring is already a single leaf. Adjust air pressure in the tire for ride quality. Most folk run it way too high, making them bounce madly.
Don't discount a used trailer found on ebay or craigslist. They are there.