What's the difference between the two styles? What are the pros and cons of each....

Which should I go with for a 24' Enclosed Trailer?

 

Doesn't really matter unless you want a Dual Cam Sway control down the road. They work best with the trunnion.

More importantly is get the right weight bars.

 

One is round is used more on the west coast and one is square and is used more on the east coast. No lie. It just cost more to make square stock than round stock.

 

Why not just get heavier bars than I'll need just to be on the safe side?

 

You want it to match your actually load capacity or tongue weight. You do not want it stiff or jerky. Heavier is not always better. It is kind of like a trailer suspension. If you have to heavy of springs or axles the trailer is going to bounce everywhere.