Open-ened question: turn raises & c/rs

I am not sure if it's the games I have been playing or maybe I am just getting a weird sample from the "norm" 1-2 6max games, but more and more I have been been seeing turn raises and check-raises when a seemingly innocent card peels. So my question is, how does one go about handling this? I will break this question down per reads:

Non-aggressive opponenent:

When a non-aggressive opponent c/rs me on the turn, I am usually looking for a reason to fold. What comes to mind is usually that the villian just made 2-pair or trips. So, how often should I be calling down with a decent show-downable hand like top-pair vs. tossing and waiting for a better line ? If the pot is sizely, I will usually see a showdown, but how good does my hand have to be against a seemingly typical passive 6max opponent? At what pot size do you decide it's worth 2 big bets for a showdown?

Aggressive opponents -

Against aggressive opponents, what should I be looking for when deciding to call down? A lot of opponents (aggressive or not) turn-raise for a number of reasons: just made a set, just made 2-pair, just picked up a flush draw, feeling spunky, their ex just called them on the phone, they just lost a big hand on another table ... etc.

For example, let's say I open from hijack with AJ, button calls and BB completes. Flop comes Q J 2. BB checks I bet, button folds, BB calls. Turn comes a 5. BB checks, I bet BB raises ...

In this situation it seems that if I were going to call down with top pair, it would make sense to call down with middle pair as well. In other words, if I have the villian beat with a Q, 2nd-pair top kicker probably has him beat as well as most aggressive opponents don't have top pair in their "check/call-check/raise" range - they will either make this play as a bluff, semi-bluff or with a monster.

I guess I am confused on the best way to handle these situations with marginal to decent hands and I would appreciate any tips on how people handle them (other than "it is read-dependent, call down aggresive opponents").