I'd likely look at the iPhone 5s given your existing ownership, although you may not see a huge benefit coming from the 5.
You face a similar problem with T-Mobile's other phones, for that matter. The LG G2 does a good job, but you won't be blown away by it coming from an iPhone 5. At least until the Galaxy S5 hits, anything Samsung makes will generally be inferior to your iPhone. And the Xperia Z1S has a nice camera, but it's both already somewhat obsolete and saddled with a lacklustre display. I'd suggest holding out until we see the 2014 smartphones reach the market in earnest, and you may want to see what the 2014 iPhone has to offer... my guess is that it drops around September, maaaaybe earlier this year.