If one would pick the 65 ats/sts over the kowa 77, for weight savings, then I think the choice should really be the kowa 550, since there's a bigger optical difference between the 65 and 77 than between the 550 and 65. Moreover, the weight saving difference is larger between the 550 and 65 than the 65 to the 77.
I think the ultimate set up is having a 550 and 770 (or 880) and decide which to take based on how critical the info from the spotter is, and if you are the stronger or weaker one in the group. If you're usually waiting up for your partner, why not arrive with superior gear at the same time anyways?
On the other hand, if it's really important to save weight, don't shoot anything.