Had both, and while its choosing between 2 evils, the htc one wins hands down. Better design, better construction, feels better, the camera software is the best i have ever used on a phone and for games and movies etc the speakers are the best.
On the other hand if the camera is not really important for you, i do love the multitasking on the s4 and the larger screen.
What can you get on the S4 that you can't get on the One? Other than gmail and the texting app, I don't use any of the crap that came with the phone. There's much better options in the play store. I have the S5 which is basically the same. I've been trying to install a custom ROM on it.