it's been discussed a bit earlier in this thread, but in my opinion the greatest advantage is audio over HDMI. For me, that is a necessary feature. The t765 does not support this out of the box. Also, I wanted a bit more power for my relatively inefficient b&w's. if neither of these is an issue for you, the t765 is likely a fine choice.
one other point that may or may not interest you is that the t775 and t785 both come with audyssey multiEQ XT processing. The t765 only has the audyssey auto-calibration features. MultiEQ XT adds room-correction, and from what I have read this is a very nice feature indeed.
according to the nad website, here is that the t765 offers via audyssey:
"Audyssey automatically detects speakers, chooses the ideal crossovers, verifies speaker phase and adjusts levels/delays."
The t775 and t785 offer that, as well as this:
"The T 785 and T 775 receivers, and T 175 pre-pro further add Audyssey's highly developed MultEQ XT Room Correction. MultEQ XT exploits sophisticated, proprietary time-domain digital signal processing to reduce the impact of room acoustics upon sonic accuracy, with a truly unique ability to yield a family sized sweet-spot, and to accomplish real gains in accuracy. Equally important, NAD collaborated with Audyssey to develop proprietary target curves that yield real-world response that reflect NAD's commitment to music first, and genuinely accurate reproduction."