The few people that have bad experiences with their phones will go to great lengths to prevent people from buying it. On the other hand, some people who love their phones will go to great lengths to make it seem that there is nothing wrong with the phone. I've read from both sides, people that experience lag, people that don't, overheating, audio, blah blah blah. I love my S4. I can say that for me, I experience no lag on my phone. BUT, I'm not using the stock launcher (TouchWiz), instead I use Nova Launcher. I have no lag when swiping through the application pages, opening or closing pages. I would say that the camera is a little bit slower than I would like (at least it seems slower than the one on my S3.) As for overheating, it does get a little warm when I'm downloading really big files (like 100+MBs), but other than that, daily things like streaming music, reading books, browsing the web - it doesn't get hot at all. OH, almost forgot, it will get hot if you use Google+ with auto backup to upload a ton of pictures. It especially gets hot when it uploads videos (mine were quite large, 1080p videos that were like 5+ minutes.)
What else, audio, the speaker is fine for me. Personally I don't like listening to music on the speakers of ANY phone, it just sounds bad no matter what - I use headphones. Besides, who else wants to listen to what you're listening to? It's quite annoying in my opinion. The one thing I do love is I have a pair of LG Tone+ bluetooth headphones that support apt-x codec and it sounds GREAT. Other bluetooth headphones/devices aren't going to sound as great if they don't support apt-x.
I've been using Android for a while now and switched to the iPhone 5 for a week - I truly missed my S3. The iPhone to me was just so... utilitarian? As in, you use it as a phone. To text. That's about it. Or that's how it seemed to me. I missed having the widgets tell me information immediately and the customization Android offers. iOS was too boring for me.
I used to use my iPod for music and my Android phone for everything else too (back on my original Incredible). Now I use my S4 for everything. I don't use the stock music player for my phone though because I have all my music stored of Google Play. I do really enjoy using the Google Music app. It could be a tad bit more intuitive, but it's very handy.
There are a lot of things you have to think about. Does appearance matter to you? Build quality? Materials used? How the device feels in your hand? How it fits into your pockets? Is it too big? Too small? I found that comparisons like call quality are really not that essential anymore as most people text anyways, but now-a-days, phones are pretty good with call quality. It's not like you're talking through cans connected by string... Maybe go to your local Best Buy, pick up the one you're leaning towards a little bit. If you're a little bit curious about the other and feel like you might have a better experience with the other, exchange it for the other.