I'm very comfortable about the Galaxy S4 that I have right now. I've had it with me for nearly 2 years (i bought it april/may 2013) and it's absolutely fantastic so far. Perfect size, good for everything I need (browsing on the bus, music, etc those sorts of things) and tbh I'm not /that/ into technology so I often don't really pay attention to all the stuff about the processor, RAM, resolution, all those things. I do care about having a god resolution, I just don't care if the HTC One X (was released alongside the S4 I think, not sure) has better colours or whatever, that's too minimal to make much of a difference.
I very much prefer Android over iOS though. The ability to actually customize UI is amazing and on iOS I really feel like I'm restricted. There's also the fact that you can even customize the OS and root your phone to install ROMs etc. etc.
I don't see myself switching phones because the S4 has been serving me very nicely so far, but I guess I could do with an upgrade a year from now at most, if I can afford it.
If you wanna switch to Android, there are a bunch of phones that are good. Nexus phones are made by Google and they're always the 1st to get updates, and they're really neat. HTC makes very high-quality mobile phones in terms of hardware, and has good speakers as far as I can tell. Then comes Samsung with the Galaxy S and the Galaxy Note series. do your research on them, they're all pretty solid.