767s are in a totally different league to 757s. You take a 767 without question if given the choice between the two.

A quick search of the AA forum will show you that opinion is usually quite divided on the 757 seat on its own. Some love it some hate it.

I personally find it quite comfortable, although its impossible to sleep in. I flew HNL-LAX-BOS-MAN-BOS-MIA-SFO without a break in April. All in the 757 F seats (except for the LAX-BOS which was in 757 Y!). All that, and I'm 6'4 too. If I can manage that, then I'm sure you can survive a paltry LAS-MIA-LAS turnaround