XTR is my top choice. Then Magura, then formula.
I have yet to find a downside to the XTR. maybe the weight, I've never actually weighed them (what?!). MT8 are just a little soft feeling at the lever. It reminds me of the Sram 9.0sl v-brake levers from back in the day. they were composite as well, and even though they supposedly tested as stiff as others, there was a sense of give at the lever. biggest gripe with the formulas is the pads run pretty close so they're more likely to rub, and in the US or around here at least, formula pads aren't often stocked.
XTR brakes are great, but in typical Shimano fashion they are not the lightest. They have great power and modulation--some of which is due to the fantastic IceTech rotors (heavy).
I just got a set of MT8 levers with a bike, but I'm planning to sell them. They are light, but I have other bikes with Formulas (equally light, if not a hair lighter) and would rather keep things consistent.
The OP won't go wrong with newer formulas. Also, word is that the 2013s have pads that retract a bit more than earlier models, which will help with setup and reduce drag. I have found it was often challenging (not impossible, just harder than with other brakes) to get to a situation with no rub. Add in lightweight rotors which are subject to more warping and you've got a problem.