Toyota 4Runner

I have a similar question. I want to know what is the longest Trailmanor that you would be comfortable recommending me to tow. I have a 2003 4Runner with the 4.7 V-8, rear wheel drive, and 5 speed auto. My only towing experiences are towing 1) a 17 ft. fiberglas center console boat with a 115 hp. motor. I was traveling behind a friend pulling an identical boat with a 98 350 Tahoe and my TV combo was "faster" up the hills because of the gearing (IMHO). I was very comfortable even at the high speeds that my crazy friend drove. 2) When my son blew a head gasket on his extended cab 4X4 Toyota Tacoma, I pulled it on a Uhaul trailer. A sign on the trailer said the trailer weighed 2000 pounds. I estimate that the combo weighed 6000+. I was not comfortable with this over 45mph on a back road. The TV would pull it, and the TV would stop it, but I wasn't real sure which was in charge- the TV or trailer. I think that the 4Runner is rated to tow 6000 or 6500.
Secondly, what advantage if any is there in towing with a 5 over a 4 speed automatic?