Hello Ms. Nadia!
A group of 14 ... wow ... that certainly is a Magical Gathering!
Umm, I personally prefer Sci-Fi and I think it has one of the best theme ideas for a restaurant in WDW, however, since you have such a large group I would say Prime Time.
Since none of you have ever been to either of these restaurants let me explain myself better!
The whole point of Sci-Fi is that you are at an Old Time Drive-In sitting in cars watching scenes from the old B movies and old commercials. With the way that the cars are set-up it may be difficult for one part of your group to communicate with the rest because you absolutely need two cars. There are tables in the center of the theater that may be able to be pushed together to accommodate your entire group but that kinda ruins the experience of the place because you're supposed to feel like your at a Drive-In.
Prime Time on the other hand is comprised of standard tables and booths which will make seeing everyone and talking with others much easier.
Here's my final thoughts!

If you don't need to be in constant communication with every single member of your group then I would certainly say choose Sci-Fi! Also, Sci-Fi normally quiets conversation because you are watching the screen.

There is absolutely nothing wrong with Prime Time ... it has a very fun theme with "Mom" and "Dad" telling you to eat your veggies and keep your elbows off the table but I just prefer Sci-Fi.

If you DO decide to go to Sci-Fi MAKE SURE you tell the girls when you check-in that you want cars and not tables ...
wait for the cars ... it will make the experience much more enjoyable!
As far as the food goes ... I am not a picky eater and I could enjoy a burger for Dinner as easily as I could for Lunch. Each offers their own unique items like Prime Time with a meatloaf meal and Sci-Fi with a really YUMMY Reuben sandwich.
I hope I helped a little bit and maybe gave you a better understanding of each. If there is something I missed or something you want more info on PLEASE ask! I don't want to plan your trip for you ... because that is half the fun ... but I want to see you make the best decisions for your group! NO REGRETS!
Take Care & Be Safe!
Frank
Disney DieHard