What do you mean by when you say they have equal surround potential? Are saying they sound similar?
No... I'm not saying they sound similar, at least tonally. I mean they both have the potential to let you hear "home theater" surround, simulated through stereo headphones. Your ears may experience the surround effect better with one or the other, they're two slightly different techniques to create the same effect. The Xonar U3 uses Dolby Headphone (2), and the Recon3D uses THX TruStudio Pro.
Tonally, bass or treble response sound a bit different. The Recon3D probably also has a cleaner analogue signal. The other sonic difference is the Recon3D has the
extra potential that if you're playing a PC game that supports 3D surround audio like Bioshock, Battlefield 2, and Unreal3 engine games like Borderlands 2, with horizontal
and vertical positional cues, you can configure the Recon3D (also X-Fi cards with CMSS-3D) to bring the third dimension to audio. Many games sadly do not account for vertical surround because home theaters just have a circle of speakers (not a sphere), for example I got frustrated a few times playing CoD4 last night when I tried to get the jump on someone around the corner, but they were upstairs.
So, if you're playing a game with only 2D surround,
then the U3 and Recon3D both only have the "potential" to create 2D surround (neither can "fake" added vertical surround without the game providing info). 2D surround is still better than stereo IMO, and most of the time it works well enough. Sometimes games, like Halo since Halo 3, simulate height in the game software, along with environmental, occlusion, and distance effects. It's not perfect but it makes a difference.