Half Life vs Sky Force Anniversary
Half Life
Half Life is the first game in the famous Half Life series that has gained a certain amount of infamy among gamers mainly because Valve has refused their fans' request to create a Half Life 3. That being said, the game itself is pretty old - released back in 1998 - and it was a shining gem of a game back in its time because of its amaing gameplay and engaging storyline. Unlike any other games in the 90s, Half Life manages to combine action and adventure along with a what-was-back-then a realistic world into one amazing package. The game also has a multiplayer mode that turns the game's many maps into arenas for you to play against your friends.Half Life is one of "those games" that almost every gamer (midcore and above) would at least have heard of before. It's as classic as classic games go. If you enjoy old-school games, Half Life is probably one of the first few games you'd want to play.
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.