http://mpc-forums.com/viewtopic.php?t=87708
here is Nym's reasons for getting OS2 which of course needs os1 to use. But i totally agree with Nym, the doors that os2 opens are uncomparable to os1. The amount of processing you can do to your sounds is, for the lack of a better word, amazing. Trust me this will take your work or beats or what ever you are doing with the MPC, unless you are just using it as a MIDI controller, to a new level. I have had it for about a day, and i could never go.
Also i am assuming in march 2008 when JJ releases the "1.0" version of JJ OS2 he will stop updating os1. This is just my thoughts no promises, but it is definitely worth it to spend the $80 and get both. You can always chose which one you boot to (hold tap tempo during boot, for your info

).