My 4Runners were/are "Gen-3", models, and not the ones which you would purchase today. Our 2005 V8 "Sport" was totaled by a man who possibly suffered a fatal medical event, slumped onto the accelerator - and hit our stopped vehicle (red traffic light) at extremely high speed.
When we switched to V6 "Sport" (now called "Trail"), we were VERY surprised to find that the V6 had just as much power for climbing 10 and even 20% hills, towing the TM. The difference is, we need to down-shift by one gear, and a lot more noise comes from the engine at the higher RPM.
I strongly recommend that you get the V6 4runner. In comparison to V8, it costs less to buy, saves a lot of money on gas, and performs nearly as well. It seems that you simply get more engine noise during climbs. V6 drivetrain lifespan seems good, although we are nowhere near wearing it out. (We do change Tranny fluid at the "severe duty" schedule.) You might note that the ground clearance of the SR5 is significantly less (9.0") than that of the "Trail" and "Limited" models (9.8"); the Highlander ground clearance is even an inch less than that (just 8.0"). Highlander wheelbase is the same, but the Track is almost an inch more skinny.
I will defer to others regarding the frame versus unibody Highlander choice. As others have already pointed out, a WDH is absolutely mandatory with either of these Tow Vehicles. They're built with a lot of overhang at the rear. Without using a WDH, the weight on the front tires would be inadequate for good steering and braking control.
