The Squaire is nice, and has a few restaurants and coffee places, but so does the main terminal. Make it dependent on the time you have.
Most likely you'll have to go through security anyway (not sure though - FRA being my home base, I haven't transited there in years), so it won't make a big difference if you go out through immigration, or on through transit security.
The advantage of The Squaire is that coffee/beer prices are regular, not the inflated airport prices you pay in the main terminal. Also, you can grab some German chocolates or bread there, at the REWE supermarket.