We've been to both. While Mama Melrose has been a "hit or miss" with us (usually better at lunch), Sci Fi is ok, it's a fun place to lunch, like a "novelty" atmosphere. If you haven't done it, you should. MM is a regular restaurant and since you're doing a signature that night I'd also go with the Sci Fi just for the fun of it and it won't be as "heavy" as MM.
Check out my review which also has a few photos of the restaurant and the food:
http://www.disneydiningreviews.com/studios-scifi-drive-in.htm