My thoughts after having a 4S for a long time and using the GS4 on and off for two months (Still have my 4S with ios 7 beta 5 so I can make some comparisons)
Size - I think the GS4 is slightly too big and my 4S is way too small. The 5 is even too small. I find myself hitting buttons by accident on the GS4 because of the large size and at times, it requires two handed operation. I still wouldn't give that up because the extra space is worth it IMO.
Screen - even the 4S is brighter and sharper than my GS4. Difference is definitely noticeable when comparing side by side.
"Today" vs "Google Now" - no comparison. Google Now wins hands down. It predicts where you are going, given past searches and gives you the option to navigate there. With that said, many of "Google Now" features are available on ios.
Gallery - I think the new gallery of ios is very cool with the way it organizes by year and event. Definite winner for ios.
Camera - GS4 has much more flexibility as compared to 4S. Even with ios 7, there seem to be many more filters and effects with the GS4. Picture quality is solid on the GS4 but IMO, it is not quite as good as some of the pics I see with my 4S. YMMV because it may simply be I haven't taken alot of photos yet to do a true comparison
Mail - for the most part, I find ios easier to use but Android has some major advantages because it uses a file system. You don't have to do workarounds to simply reply to an email to a bunch of people and attach a file. With that said, I think the interface of ios mail is slicker, especially swipe to delete. I find the gmail app on both to be awful.
OS look and feel - I like widgets. I don't have a lot but have a few I like to use. ios feels like an App launcher, Android feels more like an OS, I can make it look like I want it to or I can just leave it alone. I think ios 7 looks cartoonish and childlike especially on the 4S where there is hardly any room on the screen.
ecosystem - Apple wins this battle solidly if you have any other Apple products in the house. I have workarounds to get the functionality close (ie monitoring my dropbox folder) but it is not quite the same and you lose out on imessage and facetime. You can use doubletwist on Android which really helps with syncing music.
Notifications - Apple wins hands down IMO on this. Android notifications are horrible, especially in the lock screen (can't use lock screen widgets if you use a PIN). The workarounds currently involve turning on Accessibility services, which at least on the GS4, brings about its own whole host of issues.
fragmentation - Hear a lot about how fragmented Android is and as I am going through what I cannot do with ios 7 on the 4S (ie Airdrop) it struck me that ios is fragmented, it just isn't at the OS level, it is at the feature level.
I'm still not sure which I'lll end up keeping for the next year. I need to play with ios 7 some more and then decide. Hope some of this helps.