Mainstage and Synth for playing live

What you've described is a fairly common use of Mainstage: any number of keyboard controllers, connected via USB or MIDI to an audio interface, which is connected to a Mac, preferably via Firewire. The foot controller is pretty common also for patch changes and control signals.

I've found the built-in Logic sounds to be good, not great. If you have specific sounds that need to be top-notch, you should investigate third party virtual synths and sample libraries. I use AU-compatible virtual synths for my most important sounds, and fill in the gaps with Logic sounds.

Layering and splitting sounds is very easy in Mainstage. Patch changes are smooth, and Mainstage lets you hold notes over while switching instead of abruptly stopping the sound. I personally prefer to create one patch per song, and set up different sounds for that patch on different MIDI channels, then switch channels on the controllers while performing.

I hope that helps. I'm not familiar with any other alternatives to Mainstage but I know they're out there. I highly recommend MS. I've been using it in live scenarios for almost 2 years now and never had a problem. Good luck!