We have a 6'8" wide x 12' long trailer with 10,000 gvw we haul everthing and any thing in it. It is the low profile and has barn doors on rear vs. the conventional dump trailer/truck tailgate. The low profile handles heavy loads better than an above the wheel type. Ours is a bumper mount, but the 5th wheel version would give you a little bei more load capacity as well as increased manuverability. The low profile is easy loading because of its low height weather you are loading it by hand or with a machine, if you load material with a wheel barrow somtimes it is a lot easier only having to load them on a low profile. The only draw back that we see is the way in which a pile is spread out when you are dumping. This happens because of the short distance to the ground from the bed of the trailer. I think alot of your decision is going to be based on what is best for your paticular situation. We like the low profile and the ease of loading vs. spreading out a load while dumping.
Just be sure to get the 10,000 GVWR trailer. Those are big trailers and are easily filled to weight capacity. We rent Bri-Mar trailers from a local stone supplier here in town. We love them. They can also get pretty beat up quick if you are not careful. We spend about $1,000 per year renting them at about $50 per day. We figure for the cost and the abuse they get it is better for us to rent them.
i currently have a bri-mar 5 by 8. we put a spray in bedliner in it to protect the bottom from rust. it was an added expense but has payed off in the long run. we haul stone, mulch, etc. and it still looks great. we are thinking of purchasing a tandem axle u-dump in the spring. dump trailers are the only way to go.