The answer to both questions really depends -- if you can take your opponent's queen and he can take your's back, you're really just exchanging Queens. As with any exchange, you need to determine whether it's the right thing to do based on the current position.
The same goes for your second question -- in the position you've posted I'd take the pawn, but sometimes doing so gives your opponent a non-material advantage (such as a tempo, or a lead in development, or even positional advantages). This is generally the idea behind most gambits. You need to decide whether you're trading material for a different kind of advantage, and if so whether it's a trade you think is worth making.
here i've posted some questions that i dont know the answer to.
1.is is sound to take a queen if it means giving yours away?
if your in like the second move, and u can take a peice freely, do you or continue devoloping your peices?eg.
is this sound or keep up your devolopment?.