Battlefield 2 vs Sky Force Anniversary
Battlefield 2
Battlefield 2 is one of the Battlefield classics and rightly so. The game lets you play as a soldier in one of the three major military superpowers: the United States, the Chinese or the newly-formed Middle East Coalition. Players will have access to a wide range of weaponry to do battle with as well as a variety of vehicles. Similar to the original game, the game has a large-scale multiplayer mode - the hallmark of any game from the main Battlefield series - that can accomodate up to 64 players on a single map.With more weapons and vehicles for you to play with, Battlefield 2 is a pretty nice sequel to the Battlefield franchise. However, considering that it has not yet been reworked, unlike the first Battlefield, and that currently, there is no way you can purchase the game unless you get the hard copy off eBay or a similar site, you might be better off buying any of the other Battlefield games.
Sky Force Anniversary
Sky Force Anniversary brings Sky Force to platforms other than Android and iOS, namely PC and consoles, allowing even more players to enjoy its exhilarating arcade shooting action. It is basically similar to Sky Force 2014 on mobile and features everything you'd expect to find in the latter, including challenging enemies and bosses, large number of missions, weekly PvP tourneys, an in-depth airplane upgrade system along with a wide variety of planes to fly as and weapons to blast enemies into smithereens. In this version, you can also play the game cooperatively with a friend through the game's local multiplayer option.Oddly enough, despite its similarities to Sky Force 2014, Sky Force Anniversary is available on Android as well. Unfortunately, I'm pretty sure the game doesn't support cross-platform cooperative gameplay so you can't exactly play with your friend cooperatively if he is on the PC and you on your phone.
Sky Force Anniversary is the best (and only) version of Sky Force that you can get for Nintendo Wii, PlayStation (3, 4 and Vita), and Xbox Live, but if you want to play the game on your mobile, you can get either the Anniversary version (Android only) or the 2014 version.