[...] if you choose to use a shield, should it fly on top of your vehicle until it gets hit 3 times [...] or should it stay in the spot where you set it and don't move with your tank?
Let us hear yout preference.
There is no preference (except that some people might find the one cooler, and some the other), it depends on what other game mechanics there are.
For instance, if, say, you make the game so that the enemies try to shoot at the point where the placer's ship is currently located, and assuming that ship can be moved horizontally on the bottom of the screen just like in Space Invaders, you could scatter some shield spawners along the bottom line of the screen that need to be hit by enemy shots several times within a certain time span in order for them to spawn a shield above them or something like that.
On the other hand, if the enemies are shooting rather randomly-ish, it might be better to, for instance, have a shield that moves with the player's ship but is much slower than it.
There's really more than just the two extremes, and it's better not to focus primarily on which may be cooler or more sensational or more 'superficially' fun, but to focus on what kinds of decisions and gaming elements make the game worthwhile, for instance by giving the player a sense of accomplishment. In that sense, a shield powerup that's difficult to use can be *more* fun that one that temporarily makes the player completely invulnerable, by giving a sense of mastery to the players that are willing to learn to use it. Then, the trick is how to make that learning process itself fun, which in turn can lead to a fuller gaming experience for most players if done right.
Well, at least that's what I'd say. I'm not a professional game developer, and it's been quite a while since the last time that I've been making a videogame. This post may contain traces of nuts, make you go nuts, or may be nuts so important after all.