I always buy yardage. That way I can cut the fabric any way I want.
If you wish to work with pre-cut squares, decide which size you want to work with. If you don't want to bother to cut fabric, make sure you don't have to cut it at all.
The price seems to be about the same. Keep in mind that you get 4 squares out of each 10'' square, so you have to divide/multiply by four not by two to compare the two offers.
4 charm packs equals one 10'' stack
4x 1.72 = 6.88, which is only slightly more than 6.62