Here's an article you can use as a primer.
http://www.compacttractorreview.com/haulingtractors.aspxWhat's your tow vehicle?
What kind of terrain are you in (flat, hilly...)?
Most 6x12 utility trailers that I see are rated for 2990 or 3000#. In my state, brakes and such are required for trailers over 3000#. I'm assuming the 3500# trailer you mentioned doesn't have brakes.
I've hauled my tractor on a 3K# trailer but honestly didn't like it. Being right on the edge of the weight limit isn't comforting. Being overweight opens up a whole new can of worms, especially if there's any incident/accident. If overweight and you have an accident, your insurance may not cover anything. Also, some states will consider it a felony if someone is hurt.
I'd propose you move up to a larger trailer with brakes. It'll tow/ride nicer, you'll feel more comfortable and won't need to worry as to whether you're over the limit. Both the tractor and trailer are long-term investments. I'd go with a trailer that can handle it without being near 100% capacity.
Small minds discuss people.
Average minds discuss events.
Great minds discuss ideas.