The MOXF6 would probably be more powerful than the MX61 for sound creation. Both have assignable knobs for adjusting Cutoff, Resonance, Chorus, Reverb, Attack, Decay, Sustain, Release, Volume, Pan, and other parameters. But I believe the MX61 only lets you modify the presets-- i.e., create modified versions of the existing sounds, and save them for later recall. I can do that (although to a lesser degree) with my PSR-E433, PSR-E443, and YPT-400, so I can verify that you can create "new" sounds just by modifying the presets-- but there's also a limit to how far you can go with that.
However, with the MOXF6 you can go further than that, even loading your own waveforms if you wish. But even without the ability to load new waveforms, I think the MOXF6 would probably be more flexible than the MX61.
Have you downloaded all of the documents for the MX61 and MOXF6 from Yamaha's web site? They should help you get an idea of what each keyboard is capable of. Also, are there any music stores near you that have both keyboards on display so you can compare them? At the very least, see if you can test drive the MX61 so you can get a feel for whether or not it would be ample enough for what you want to do.
Coincidentally, I was just reading a thread on another site where a veteran keyboardist was saying how impressed he was with the MOXF6 when he got a chance to play one at a friend's house. I know of someone else on yet another site who was debating whether to get an arranger keyboard or the MOXF6, and he finally chose the MOXF6 because he was interested in creating his own sounds. The last I heard, he was very happy with his choice. So you probably wouldn't regret getting the MOXF6-- although you might regret getting the MX61 if it turned out it can't do as much as you'd hoped it could.
