I'd say to go with the 2.5" cut size. If nothing else you can always cut down but you can't make the small cuts bigger. Recently I found a bunch of unused Y2K squares and started looking at options -- there are a lot! So many that I could probably spend the rest of my quilting life doing nothing but working with 2.5" cuts.
The 2" finished design unit is a very common, 1.5" is less so.