Portal 2 vs Sky Force Anniversary
Portal 2
Portal 2 follows in the footsteps of the award-winning original while continuing the story of Chell and Glados. Now with both single player and a 2-player cooperative mode, Portal 2 ramped up its game with a plethora of new, challenging puzzles, a storyline that's a lot richer than before, and more advanced and realistic physics that will govern your gameplay. Interestingly, the game also includes editing tools, allowing players to create their own puzzles which can then be shared for all to enjoy.Portal 2 is definitely a much better game in the series and if you fancy giving this franchise a chance, you might want to jump straight to Portal 2.
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.