Is mainstage the right software for me?

I actually use both Logic itself for this, and also main stage. I play drums in a youth worship band and sometimes we have the whole intro to a song tracked in Logic, but I also record activate 2 pads and a piano sound and have the sliders on my Axiom set so I can adjust the levels of each. The loop plays and runs out, but if we do a slow ending, I can switch over and play keys.

MainStage has had performance issues for me, but so far from what I've seen, the setup is pretty sweet. It's certainly going to be more robust than a small midi sound box, and the fact that you can program all the dials and sliders on your midi keyboard to actually do something (and it shows you visually on screen too) is a big deal.

The key element in MainStage is that it allows you to map MIDI control to software plugin settings. A secondary element is that it's pretty; i.e., has a pleasant visual interface. The fact that it's bundled means two things: one, that you get a pile of plugins and sounds to get started with, and two, that you're buying a lot of other stuff you might not need.

If you go looking, you might be able to find digital audio workstation software that allows you to map MIDI for less money. I'm sorry I can't give you a definitive list, but a quick glance makes me think that, for example, Ableton Live LE might do the trick. (The set of possibilities expands if you aren't necessarily committed to the Mac platform, but we won't go there.) In such a case, you're going to need software instruments; you can pick and choose the ones that seem like they're going to give you the most bang for the buck in your situation.