Ignore the tools posting above. Not sure why you can't ask a simple question and get helpful answers around here.
I'd buy the one that shows a dyno chart with the highest output. I doubt you will find anyone that has purchased more than one tune and will be able to offer advise. European Car Magazine often does tunes on vehicles and posts the results, but they generally don't do several tunes from different manufacturers. Not sure if they have done the M3 or not. You should be able to search via Google for that.