both do similar things except Samplitude pro is more a PROJECT based enviroment vs Samplitude Magic Studio which is At the Moment (NOW) recording and modifications
the Pro is more based if you are going to be doing some MAJOR projects with different bands, groups and want to keep there settings seperate
Music Studio is basically a "lite" version of Samplitude. You don't have access to some of the tools like the Routing Manager (all inserts & Aux sends are post-fader), Audio Quantization, the Take Composer, etc. Also, you don't get the fabulous plugins that are included in Sam Pro (Am-munition, Am-track, etc.). And of course you are limited to 8 Stereo/16 Mono simultaneous ins and outs, limited on track count, VST plugins per channel, VSTi's, etc. This is a few versions old, but i think most of it still applies:
I considered upgrading recently, but decided against it. I'm still able to do pretty much everything I need to in my project studio using Sam Music Studio 14. One thing to consider though--if you buy Music Studio and decide to upgrade to Sam 11 or Sam 11 Pro, you are eligible for upgrade pricing, which is about a 50% savings over the regular price. Most any Magix dealer can sell you the upgrade--just search online for "crossgrade from Samplitude SE/Music Studio".