Unlike Nintendo, Capcom has no problems with releasing games for smartphones. Oddly enough Final Fight was released on the Capcom Arcade app which was seen earlier this year, so we have no idea why Capcom has decided to re-release it as a standalone game. If you already own the Capcom Arcade app, then perhaps you could skip Final Fight and go straight into the Street Fighter II Collection.
As the name implies, the Street Fighter II Collection is a collection of various versions of Street Fighter II, and will include Street Fighter II, Street Fighter II: Championship Edition, and Street Fighter II: Hyper Fighting. No online multiplayer available, unlike Street Fighter IV Volt, but there is local Bluetooth multiplayer, which I personally find more fun.
Final Fight appears to be on discount and will go for $0.99, which will end on the 21st of September. Street Fighter II Collection is priced normally at $3.99 and both games are available on the iTunes App Store.