This is a no brainer if you do not miss the nuts or the Admiral's Club access.

The 757 First is leagues below the 767 Business. I flew SFO-BOS Business class recently and fly SJC-BOS 757 First often (all upgrades). The 767 seat has a foot rest, recline well, have an adjustable lumbar support and a better pitch. The 757 seat are just an wider and thicker version of the coach seat. More plush and more leg-space but that is it, no leg support, no lumbar support, limited recline. I have never been able to sleep in the 757 seat but I slept very well on the 767. The seats are definitely better on the 767. I am not sure whether I flew the 767-200 or the 300 though.

Also the 767 transcon has more menu choices, more choice on the wine menu etc. The only thing I missed were the warm nuts. Of course Admiral Club access is another issue, but only if you need access during a layover or before the flight.