Current iPhone user looking to transition

I've been using my trusty iPhone 3GS for the better part of the last four years and have been content, for the most part, to stay put. However, certain issues have slowly cropped up that are increasingly unable to be ignored. Cracks are creeping up the back of the phone from the 30 pin port to the FCC logo. The speakers are getting weaker every day, the screen is scratched to hell and back and the headphone port is slowly falling apart from the inside. Not to mention the software is no longer supported.

It chugs heavily in Safari and any game other than Bejewled or Angry Birds. Opening Apple's podcast app or the camera brings the phone to a 10-15s crawl and the battery takes a sizable hit on even menial tasks with minimal screen brightness.

All that said, it holds up for what I use it for: lots of calls, light text messaging, checking email, listening to music on the rail and way too much reddit.

But, make no mistake, an upgrade is long overdue.

I would pick up the 5s, but the off contract price is very prohibitive and I'm not keen on getting locked into an expensive contract for an extended period of time. I've been spoiled by MVNOs like Straight Talk.

I'm looking toward the Nexus 5 with fingers crossed for a moderate off-contract price, but I'm very apprehensive about the idea of a 5 inch screen after years of dealing with a relatively svelte 3.5 inch device.

However, the recent announcement of the Moto X on Republic Wireless has me intrigued. I love the price point for the phone and plans and Sprint has solid 3G (though no 4G) in my area. The voice control and active display sound excellent and I like that the form factor packs is smaller while maintaining the experience and delivering solid battery life.

Any advice for moving through a platform transition?