The Samsung Epic 4G Touch (Galaxy S2) is coming out September 16th and will be the most powerful phone on Sprint. I would put my money on this.
Pros
-4.52 in. Super Amoled Plus display (With Gorilla Glass for an extra durable) which is arguably the best display on the market and saves more energy than LCD screens. (The phone's size is barely noticeably bigger than the EVO 3D)
-Thinnest SmartPhone on Sprint: 9.59 mm vs 12.05 mm (EVO 3D)
-1.2 GHz Samsung Exynos Processor w/ Mali-400 GPU (The most powerful chipset in any phone to date)
-1800 mAh battery
-Fastest Mobile Browser
-8mp Cam w/ 2mp FF Cam; Shoots High Profile 1080p HD Video (Arguably the best phone camera on the market)
-Supports High profile MP4, MLK, DivX, Xvid, etc. video playback formats
-TouchWiz 4.0 UI, with more functions than any other UI, including Motion Features
Cons
-Samsung's support has lacked on the previous SGS but from the SGS2 forums I've read through, they have been getting a lot of support for this phone.
-No Kickstand (Not that important but kinda handy)
-WVGA Resolution (800x480) vs. qHD resolution (960x540) on EVO 3D and Photon, but the Super Amoled Plus display more than makes up for this
Here's Part 1 of the Wirefly Review:
Samsung Epic 4G Touch Review - Part 1 - YouTube
A Hands-On video that shows the Motion Features (Skip to 1:35):
Sprint Samsung Galaxy S II Epic Touch 4G Hands-On - YouTube