I would suggest lunch. They are not known for having great food. You are going for the atmosphere which combined with something simple such as a burger or such it's a fun and pretty good lunch. If you go for dinner you get into dinner prices so you will end up spending more and it's just not worth it in my opinion. Besides, it's night time inside regardless whether you have lunch or dinner.
Rudy, keep it simple at Sci Fi. Bergers, appetizers and shakes are all fine at SF. I would not recommend their dinners. Go for the ambience and lunch fare.
Sci-Fi is one of my favourite eateries: but I will not go there for dinner any more. Lunch is the only way to try this establishment. The burgers,onion rings and milkshakes are excellent, imo the best at any WDW eating spot.
I have read here that you can still get a burger for dinner even though it is not on the menu.(dinner there is not highly recommended here on the disboards) Maybe that would help you if you are interested in eating there at dinner time.