I have been to both and like both alot. They are different, I think, and hard to compare. FOr one, conversation is not really easy at Sci Fi. You're set up facing a movie screen (like a drive in) and people watch films while they eat. I think its 2... maybe 3... people across. So you can talk easily to people next to you, but have to twist around to talk to people around you. So if youre looking for family conversation, you wont be able to do that easily there. The food is different, but good, at both. SciFi is more burgers at lunch, while primetime is homestlye cooking - both delicious!!! Primetime is alot of fun if you like the potential of being teased, and get a good server. You can easliy have conversations here, but know there is usually so much interaction going on between tables and waitstaff that even that is not that easy =)
Dont think I really answered your question, just wanted to point out they are diff experiences depending upon what you are looking for. Theming and food is great in both, I think =) I'm sure you'll be thrilled with whatever you decide!!!!!!