[Reynolds]

What Ranma said, mostly. There are a ton of factors to consider when moving beyond that scenario - how you play, how well you're playing that day, the type of teammates you have, the short term goal of the map, the long term goal, if you're looking at pushing through bodies or entrenchments, what the class make up is on both teams

For me, the things I generally look at the most are the: Terrain - ie, defending dustbowl 3-2, unless you're pushed to the point, kritz is almost always amazing; Attitude: Aggressive teams make better use of kritz than non-aggressive teams; Class makeup - if there are 4 scouts, 3 snipers, 3 spies, 4 pyro's, that kritz is going to do way less for you than an uber.

That said, almost every single 'rule' for being a medic evolves like this (really true for every class, but it seems like there are more 'hard rules' floating around for what is supposed to constitute a 'good medic':

Action A is awesome, I'll do it all the time either intentionally or unintentionally!
Only bad medics do Action A, so I won't do it!
Really good medics avoid Action A, but know when to do it anyway.

In short, grab whatever gun is going to make you win. If you're in doubt, go with the normal uber