OK decision made: just bought my ticket, UA/A3 SFO-FRA-ATH-FRA-SFO, 3 hr connection time for both inbound and outbound, United operates w/ 777 those flights i was able to get the 17A both ways which according to seatguru should give me lots of legroom so i opted to buy cheapest non upgradeable and save miles for a business class award ticket SFO-NBO later in the year.
Well done! ^ Seat 17A on the 777 is one of the best coach seats in UA's entire fleet, with nearly as much leg room as the 777's business-class seats but with nobody reclining in front of you. I think you will be very happy with your choice.