This is not an IQP yet, right ? 
Both developments are playable (Bg4 or Bb7), but if you play Bg4, you don't want to trade on f3 (the only exception might be if you can anchor a strong Knight on d4). You just keep the pressure and wait for white to commit.
Tha advantage of the pressure with Bg4 is that white must be careful about his d4 pawn, and his LS bishop can't take an agressive posture on d3 or c4.
Here is an example from the regular Alapin :
I had this position in an OTB game:
(I was black)
that is basically an Alapin a tempo down.
The decision I had to make was to either develop my lsB outside of my "pawn chain" (e6), or to leave it closed in by playing e6.
I went for Bg4 because I thought it made sense to pin and eventually trade the knight on f3, because it's the main defender of the IQP. However my opponent simply played Be2, and when the trade occurred his bishop on f3 was really strong against my queenside weak light squares.
In the post game analysis he told me he just didn't like this move, because of the weaknesses left behind on b7.
So, in general what should I do with my light square bishop against the IQP?