I must chime in to add that the UltraLite mk3 (I am using the older FireWire only, non-hybrid only version) has been the best of all worlds for my MainStage setup. The I/O flexibility is great, the latency is minimal, and the hardware DSP (reverb, eq, limiting, compression) has been really useful in offloading CPU tasks from my MacBook.
An added bonus is all of the metering stuff you can use to analyze your signals, which is sort of a hidden benefit of the box.
I've tried an RME Fireface, an Apogee, and the MOTU, and the MOTU has been ideal for live performance.