Good question Linda. You would have to look at the features to see what you'd really miss and what you would and wouldn't use really.

The GW-7 has more poly (64 I think)
The GW-7 has user slots for custom voices

The MM6 soundset is based on the Motif Classic
The GW-7 has its own soundset, but also has a lot of ethnic voices too

Both are limited in their sequencers. The GW-7 will give you 16 tracks though where as the MM6 only gives you 8

The GW-7 will function better for more traditional styles. The MM6 is geared for modern music

Neither unit will allow you to record user styles

I think construction quality on the GW-7 just might have the edge on the MM6

The MM6 is going to have mega voices, and it has some good acoustic guitar arps too.