If you like putting custom roms and experimenting new root apps features. then I would highly recommend you i9505. If you are never going to put custom rom and i9500 is cheaper than i9505 at your place then only you should get i9500.
I have never compared custom rom battery life with stock rom battery life. but I can tell you that there is a difference.. Custom roms provide better battery life than stock roms. because stock roms have so much bloatware apps installed which are always running and sucking battery juice. Also there is an app special for snapdragon powered devices called Snapdragon™ BatteryGuru. I have never used it. but it has some good reviews. It can be useful.
I am a i9500 owner and I hate this phone so much. because it has no good custom roms like cyanogen,pacman,paranoid,aokp,aosp etc. because samsung has made exynos chip source code closedsource.
I dont think there will be much difference in terms of app performance. Both can run apps equally smooth and good.
Btw why dont you get nexus 5 32gb? I guess both s4 and nexus 5 has same price tag. In terms of performance nexus 5 is far better than s4.