No problem man.
There's a comparison chart on the website that makes it real clear what each OS offers and in rare cases what you might lose to gain one or more features. Usually pretty good trade offs.
It won't take long using the MPC to be able to use the chart to see what's worth the extra investment to you. Most of the features as you go up the different OS's are things that you'll probably think of using Akai or Free JJ and say to yourself "it would be great if I could..." The XL/OS3 versions the answer is usually "sure we got that, just toss some $ over to Japan".

And the old ones are no slouches either like Crossings mentioned.