If you are a looper - and that means most players from the intermediate level on - then when someone pushes to you and you want to loop, you should be in one of four mental states. We're assuming you can loop both forehand and backhand, or (if no backhand loop) have very fast feet for #4.
1) Loop forehand or backhand, depending on where the push goes. This means there's essentially a line from your opponent to your playing elbow (mid-point between your forehand and backhand), and if the ball is on the forehand side of that line, you forehand loop; if on the backhand side, you backhand loop.
Advantages: You are ready to loop anything, and you don't have to go out of position.
Disadvantages: Your opponent chooses if you are going to loop forehand or backhand (and presumably will have you do your weaker shot), and you might have problems deciding which way to go on pushes to the middle, especially quick ones.
2) Favor forehand, but ready to loop backhand if it's a strong push to the backhand.This means you are ready to forehand loop most pushes, including ones to the middle and weak ones, but won't force it against a good push to the backhand, which you'll backhand loop. (Of course some might just push this ball.)
Advantages: You are ready to forehand loop - presumably your stronger side if you choose this strategy - against most pushes, both strong ones anywhere except to the backhand, and weak ones anywhere.
Disadvantages: You may have a lot of ground to cover on some shots if it's a good push toward the middle. If you are ready to forehand loop and are forced to backhand loop, may not be ready and so may backhand loop weakly or inconsistently.
3) Favor backhand, but ready to loop forehand if the push goes to the forehand side.This means you are literally setting up to backhand loop most pushes, including pushes to the middle or even slightly toward the forehand side, but are ready to rotate the shoulders to the forehand side to forehand loop if the push goes there. You are basically telling your opponent, "I'm going to backhand loop, but if you want to give me an easy forehand loop, then push to my forehand."
Advantages: Allows you to really prepare for your backhand loop, often compensating for having less power on that side. Also allows you to have less ground to cover for forehand loops. Allows you to stay in position for most loops. If your backhand loop is stronger than your forehand loop, then allows you to maximize the chances of backhand looping.
Disadvantages: You'll be doing a lot of backhand loops, often weaker than the forehand loop. Can be caught off guard with a quick push to the forehand side if you are too quick to set up for a backhand loop. Can have trouble backhand looping pushes that go to the middle if you don't learn to step into position properly for this.
4) All-out forehand looping. You basically decide in advance that if it's humanly possible, you are going to loop with your forehand. Can only do this if your footwork is fast and technically good. Off the serve, good footwork technique and anticipation can make up for not having fast feet. (Note that the logical alternative to this, all-out backhand looping from all over the table, is rarely done since there's less range on the backhand side. There are some players who do this, but they are rare.)
Advantages: You get to use your forehand loop, presumably your stronger shot. Allows you to get into forehand position so you can do a series of forehand loops in a row. Takes the indecision out of the shot since you know what you are going to do.
Disadvantages: You have a lot of ground to cover, and so can get caught out of position, both while trying to make the shot and for the next shot. May make weak or inconsistent shots if you aren't in position quickly enough.
Conclusion: Which of the above do you use?