I pick upgrades in this order:
1: Do I really need it or can I hold out?
[if it gets me no closer an upgrade I really need/want, try to save XP]
2: If yes then choose the one that leads on to a future upgrade.
[no brainer! If it's needed to get another upgrade, then get it]
3: If neither are steps to something else, I go for penetration over damage.
[guns with higher penetration generally work better at range]
So, by my criteria, I would probably go for the 76mm, unless the other gets me closer to an upgrade I want or I think I can hold out a bit longer.