I considered the Highlander before getting my 4Runner. The main sticking point was that the back seats of the Highlander didn't fold down in a way that is best for our dog; 4Runner is better: i.e.. lower seat first; back rest sits flat, thereby no gap for the dog to "fall" into. Also, the Highlander is more car like and has more glitzy "city" finishings. The 4Runner seemed more truck and sport based styling. In retrospect, I am so glad I have the extra interior space of the 4Runner. We can put 2 small (enduro/trail whatever) bikes easily into the back with the seats folded down and otherwise have tons of space for passengers, our dog, and sporting gear etc. We live on a hill so our mileage will reflect that at 12.7L/100km. It drops down significantly on the flat and especially with highway driving. It's pretty similar to the Toyota Tacoma for fuel mileage.
Not sure what you mean by the back seat gap, my 2013 highlander has flat folding seats so the whole back area is flat, I sleep back there at bike races, and the cubic feet comparison shows 94 cu ft for the highlander with back seats down vs 89 cu ft for the 4 runner.
I agree the 4runner has a more truck like feel and burlier frame, but the highlander has pretty good ground clearance and even hill assist descent, so it's been pretty good on the forestry and logging shuttling roads with much better fuel economy and same hp numbers as the 4 runner.
Are you talking about the older style highlander, or the new 2014 model?