If you are a party of 6 you can all fit in one car at Sci Fi. It is a very unique and fun atmosphere. The food is good, not great, but certainly worth the experience. H&V is just another character meal although not sure how old your kids are, if they like the characters that appear there it could be a better choice.
Feel free to check out my review of Sci Fi, it is a little dated but gives you an idea of the atmosphere, etc.
http://www.disneydiningreviews.com/studios-scifi-drive-in.htm
If I had to choose between the 2 I would go for Hollywood & Vine.
I am sorry but we have been to Hollywood & Vine three different times and it has never been dirty and we have always had good service. The food is nothing to rave over but it is good and will satisfy.
Sci-Fi - next time someone is there take a picture of the dark booth and see all the food stuck to that booth you can't see in the light. It is gross.
I personally feel the food is just as average as Hollywood & Vine.
Sorry I am a bit of a germaphobe & if I see dirty bathrooms, menu, or seating areas, I will not return to a restaurant in the real world or at Disney.
My vote is for Hollywood & Vine