Being that one of your two choices doesn't actually exist yet, with completely unknown specs and features, it's kind of hard to give a recommendation based on one of the phones being a big question mark right now.
That being said, a few thoughts:
- if you don't need to replace your phone by April, wait until the iPhone 6 is announced a few short months after the S5, and see which one you like more. Apparently the new iPhone will have a bigger screen (according to rumors)
- have you ever owned an Android phone before? Do you have a need for the features that Android devices have over iOS devices (specifically widgets, sideloading apps not on the app store, access to the file manager being typical items). If so, then you might be better off with the S5. If not, then not.
- do you like getting your phone updated in a timely manner, or are you fine with waiting for updates to trickle down through carrier approval before getting updated. Do you want 3 years of reliable updates, or do you not care about getting the latest OS updates in a timely manner. If you like getting your phone updated, then you do NOT want a non-Nexus Android phone. It's one of Android's biggest Achilles heel's. Here's a good article on iMore's sister site talking about the issue:
Solving the impossible problem of Android updates | Android Central
For my personal opinion (which is mine alone, and might be considered bull-crap by others):
- I've had my toes in both ecosystems. Android devices do have a bigger feature list (like the ones I mentioned above). However, the lack of updates to existing devices, lesser app selection (most apps come out for iOS first, then Android months later - if at all) and other issues started to wear down on me. The gee-whiz factor of widgets and being able to customize your home screen with different colors and icon packs wears off real fast, especially when the OS overall is laggier and has more issues than the 'less featured' iOS. My Apple devices have just plain worked better than my Android devices, which is why I switched. Also, since my family has several Apple devices between us, the seamless-ness in syncing data between the devices is a big plus.
I'm past getting into flame wars over what mobile OS a person likes, however that's just my personal experience and recommendations. Your needs may be different than mine.