Ive recently became eligible for an upgrade I have the S3 and love it I could get an S4 for .1 cent and its so tempting from all the reviews I've read it seems to be awesome. But there's also a rumor of an S5 being released around march...So maybe with the upgrade it will be affordable enough ill pick that one up.
Announcement of the S5 is expected soon (Q1). Current rumor is there will be 2 versions; a standard version with impressive hardware and the typical plastic construction, and a premium version with even more impressive hardware and aluminum construction.
The S4 is not the fastest phone, but it's still "fast enough" for anything you'd want to use it for
today. Its processor/GPU is a step down from the current top-of-the-line, and 2 steps down from what we should be seeing very soon. The fastest phones right now are the Nexus 5, the LG G2, Sony Xperia Z1(S), and the Note 3, but they all have one or two drawbacks that prevent them from being considered "perfect". The Nexus 5 and LG G2 don't have expandable memory. The Nexus 5 only works with AT&T and T-Mobile. The Xperia Z1S is only available from T-Mobile, and is a bit large for its screen size, but otherwise is an amazing device. And while the Note 3 is at the top of specs and performance, it's also very expensive (even on upgrade), and is huge size might be too large for many.
The S4, although slightly outdated (and thus, not quite as "future-proof"), still has many benefits over other current flagships. MicroSD expansion for one. Replaceable battery for another. There's lots of accessories for them. and being one of the most popular phones, you're less likely to come across app bugs or incompatibility, and see continued software support from Samsung. Currently, with a new contract or renewal, they can be had for only $50-100. It has a great camera. I definitely prefer the physical "Home" button and capacitive "settings" and "back" button over the on-screen buttons that other phones have (which seem backwards to me).
Of course, phone choice also depends on your service provider.