Either way you do it, try and connect the supports the the boat's ribs or seats, not directly to the hull .... if you can help it.
With that said, I'm a big fan of welding. You can get a bit more creative with your design since you aren't limited to creating good rivet placement spots. I also think welded support structures are cleaner and usually require less material.
Granted, rivets can be removed more easily than welds, but that shouldn't be a big issue.
I would, however, rivet the decking down to the supports instead of welding it down .... unless you are 100% sure you won't ever remove it.