They both have their ups and downs, even though Squirrel has more advantages and less disadvantages than Pawn does.
I've noticed that Squirrel executes faster than Pawn, although this was tested from the LU server executable versus the SA-MP server executable.
One major disadvantage that I don't like against Pawn in the whole Squirrel/Pawn comparison is the array capabilities in Pawn. They really suck ... badly. Squirrel arrays can be anything from a function to a string or integer or even another array. Also, Squirrel arrays can be multidimensional, as Pawn is limited to 2 or 3 dimensions or something like that. I also hate that public functions can't return strings in Pawn.
The only upside of Pawn versus Squirrel in this case (besides the precompiling and "closed source" capabilities, which is available in Squirrel too, I think) is the amount of scripts made for it, even though this isn't really due to it "being Pawn", but more or less because the amount of support from the SA-MP community.