I'd definitely recommend Sci Fi.
Personally, I like the food at Garden Grill, but I'm not so keen on the atmosphere. Feels geared toward the retiree crowd, with the exception of the characters.
Sci Fi, on the other hand, is completely unique, unlike any dining experience you'll find not only at WDW, but really anywhere I've seen. Plus the food was excellent when we went. My wife said her milkshake was the best she's ever had.
The kids loved sitting in the car and eating, my only complaint was that at times it was too dark to see my food (was trying to pick around the zucchini in my pasta dish). But it was worth it.