do you have a DAW? if you do, go with the MPK49 IMO. even if you dont have a DAW, I have heard a lot of bad thing about the mm6 sequencer. If you want the sounds, look into a motif rack, its more expensive but probably a better value.
so the real questions are
do you have a DAW, if so, what
do you want sounds, or just a midi controller
here is a user review from zzounds of the MM6 (keep in mind this is a particularly negative review, however it does bring up some good issues):
Feature:
Some of the functions sounds good like layers,cutoff,reverb,phaser,flanger and etc.But it is missing the most basic sequencer functions you can't set the measures or quantization,swing,sections
Quality:
For the price it could of been alot better than a over-priced usb 61-midi controller that doesn't automap or have any drivers to map the surface that happens to have sounds.
Value:
I was mad after like hour of playing it.
Sound:
Its has good sounds but the patch banks are small.It isn't a mini motif tho my friend has one and its a beast,but for somebody that doesn't want to edit your voices the mm6 for you.The TRS 1/4 outputs sounds like its less than 16-bits
Ease of Use:
Well the manual is only about 20 or 30 pages its pretty easy
Support:
Now the support was great from yamaha and steinberg