IPhone 4 or 4S? Better ios 7 performance?

I recently decided that I would like to switch back to iPhone ( after 6 months with Android ) I finally saved up enough money to purchase one on my own. But I was wondering whether I should be frugal & get the iPhone 4 ( I heard it's really slow on ios 7 though) or hash out a little more cash for the iPhone 4S. What I really want to know is if there's a big enough difference in performance, specs & overall ios 7 experience for me to spend more? Thanks in advance for any help & suggestions, I appreciate it.

IOS 7 is good but I'm not sure I would install it on an older version, I would probably only install it on 5 or later. There were alot of sales over Christmas for 5C on contract for $49, etc, why not go that route? (4S is free on contract).

Ios7 currently lags per say on anything under than a 5. Unless they fix the problem (which is unlikely) don't upgrade.