Why I wouldn't buy a Galaxy S4..
Lets look back at those of us with a Galaxy S2. Which is just now getting jellybean. The jellybean update is bloated by the way and makes the phone a little slow compared to CM 10.1
Also, speaking of CM 10.1 Samsung won't release the proper sources and therefore, things are missing from cm 10.1 . Samsung will not release Exynos source code! (see here http://forum.xda-developers.com/showthread.php?p=37588626)
So what does this mean? Well, when you buy a galaxy s3 or s4 you are at the mercy os samsung. When the rest of us get the latest updates on the Nexus line of devices, samsung will be lagging behind.
Also, touchwiz has become horrible and bloated more than ever. Sure you can use a different launcher but that doesnt mean its still not there wasting space and making things difficult.
If you are a fan of android updates, proper , within a decent time range i suggest a Nexus 4, or Sony Xperia Z . I wont even think about the Htc ONE since they seem to be slipping with proper support