Sci Fi al the way. While I can see how it might stink to sit behind someone else, in the end this place is about the drive in movie experience. Everyone is riveted to the screen. Sci Fi is the one and only WDW eatery we have made ADR's at each of our trips (had to cancel one time due to a family member being sick and we were crushed to not get to go and break our Sci Fi streak!). We love the burgers, shakes, reuben, fries, onion rings and real made to order cherry cokes.
Its generally quite and relaxing. In spite of the movie clips playing, you feel relaxed because the diners (and most of the kids) are quiet. Just eating and watching!
H&V is my least favorite place I have been to in 12 trips to WDW. I agree with the poster who said they'd only go there if they had kids dying to see those characters.