Which is better for a base?
+5000 Shield(1 Rift Grid)
+2500 Hull(1 set of Nexus Base Plating)
+800 Defense(2 Phase Shift Arrays)

Defense, then shields. Or the other way round, if you'd prefer.

Depends on what player races and professions you have in your legion. It also depends on the currents stats of your base. So without direct information, my advice would be purely speculative.

That said, I'd probably go for the shield. Normally I pick hull because hull gives you more hit points for your buck. But your hull is weaker than your shields so that argument is tossed aside. Defense and hull are usually the bulwarks of turtling a base but adding two phase shift arrays won't hugely increase the defense of your base (unless it's already low). So of the modules listed, I'd go with the shield. Cause 5k shield is 5k shield and annoying to have to get through and then keep from regenerating.

I've already said that I don't know what the stats are on your base so I'll throw the stats for my base out there. This is the kind of ratio I'd work towards... and if I were to put another module on, it'd probably be another meta chron shield.
Level 4
31830 / 31830
Shields (meta chron)

76945 / 76945
Hull (fermionic)

Attack 3250
Defense 26787 (12 trans nacelles and 2 comm links)