I just pre-ordered an M Audio Code 49 controller to use primarily as a mixer during live performances with SPLAT as the "mixer." After reading about issues with mapping on various forums, I am concerned that a Nectar or Roland keyboard controller may be the way to go. 
 
I don't mind spending time mapping the individual faders knobs and buttons manually if need be, but it seems that others have been pulling their hair out. 
 
Ultimately, I want to have a reasonably small and versatile controller on stage to tweak the mix of the vocals on the fly, trigger a few fills and/or loops and play simple keys behind the keyboard players solos.  I would like to have presets for when the drummer has a lead vocal song, keyboard player has a lead vocal and so on, so I can just change to a preset with their particular vocal being prominent.  I don't need to exploit every nuance of the keyboard controller, just map the basics to get rolling and be confident that their won't be a meltdown on stage.
 
Any thoughts on mapping or a good choice for a keyboard controller?  Thanks everyone for your time!

Hi. If you don't mind some mapping, I would suggest the AZ Controller, made by fellow SONARian azslow3
http://www.azslow.com/index.php
This can be used with any(?) MIDI controller.
He might chime in on a much higher level than I am able of. 
Others will know more about MIDI keyboards in general than me.
All the best.

Ken has already explained how it goes.
 
I just can add M-Audio Code specific info:
* it has 2 modes (switchable on the fly, also per control group), MackieControl and MIDI
* MackieControl is fixed, not really documented, layout which imitates Mackie. With my mod of MackieControl (more stable) or Mackie preset in AZ Controller (almost untested but freely re-configurable) you can get normal set of mixing functions in no time. Just aware that stock MackieControl supplied with Sonar will not work.
* MIDI mode layout is configurable with M-Audio editor. You will need to create your own preset, but you can define you own needs specific configurations, f.e. with a kind of "macro" sequences (AZ Controller can send arbitrary computer keyboard shortcuts), specific tracks / effects parameters hardcoded mappings, etc.
 
For meltdown on stage... I provide no warranty of any kind. I recommend to test any function before using it on stage, I mean "last minutes" changes in the configuration can be looking for troubles. There are several Control Surface related inconsistencies in Sonar which are easy to overseen in a hurry (all known by me problematic points are documented on my site, what is know to produce Sonar crashes is workaround-ed, still there can be something I have not noticed so far). 

Thanks for the input.  I see you are dedicated to integrating control surfaces for sonar.  Truly commendable.  I saw on another post that no user provided a preset for the M audio Code series for you to work with.  Not entirely sure what that means.  However, when I get the M audio Code 49,  I'll gladly participate is sending you whatever information I can, presets or whatever else might be useful.  I'd like to reciprocate if possible.  I'll download the AZ controller software.  All the best!

Just an afterthought, but consider that Sonar may not be the best tool for your live performance.  It is coded to be a high performance studio application, but there may be better options for live use. 
 
There are Sonar users that are aware of this challenge, and are using another DAW for live performance. Some users can successfully use Sonar for backing tracks, but for interactive use (tweak the mix of the vocals on the fly, trigger a few fills and/or loops and play simple keys behind the keyboard players solos) you may be exceeding design limits.
 
Ableton Live seems to be the top choice for live performance use.  There are many controllers with dedicated control functions that are proven to work with Live.
 
There are probably other users that can share their experiences.