If you have money to burn and the only thing you know is that you want a digital synth, then you can buy either without thinking any more.
If you want to be a little more considered, then maybe asking some kinds of questions of yourself would be helpful…
Why do you want a new synth? Is there something lacking in the music that you are making currently? What benefit would a digital synth bring you? Do you simply want a new toy to play with (which is OK if that’s what you’re into)?
Then:
Do you need a keyboard?
Do you want a single-timbral monophonic/paraphonic synth or a multi-timbral monophonic/polyphonic synth?
Are CV/gate inputs and outputs useful for your set-up now or in the future?
Do you want extensive effects routing possibilities?
I believe that the Pro2 sequencer can sequence external MIDI and internal parameters simultaneously, but the implementation is somewhat different that the MM’s sequencer. The manual for the Pro2 is reportedly quite sketchy for this, so you might ask some Pro2 owners at dsiforum.com or dsi-lifeboat.com for more information.
There’s plenty of information here and in the MM manual for you to get your teeth into about the MM.