From what I heard, T&co has stricter guidelines as compared to GIA.
If you have the budget, I would say, go for a smaller stone but with excellent cut from T&co :)
I have a T&co engagement ring, and am really happy with it & am sure ur gf would too..
Although for the same price, I could've gotten a larger stone from other jewelers but after actual comparison, you can actually tell the diff in quality, not only of the stone but the platinum setting as well ..it's smaller but more sparkly cos of the excellent cut.
Have fun hunting for the ring!