I didn't do a ton of comparative research, but I tried them both for about 15min each. They seemed to do about the same thing, but Ducksboard was friendlier aesthetically and everything on their site, from the UI to the docs, seemed simpler than Geckoboard. Seems like they both offer good products, though.

Oh, I also met the Ducksboard guys at Gluecon, so they got the opportunity to explain how it works to me in person. That extra exposure and personal connection probably influenced me quite a bit too.