Personally, I've used both, but settled on SureSpot for the moment. SureSpot uses data exclusively, which is cheaper than SMS for me. Although I understand that TextSecure now has (or will be getting soon) a data channel. So I'll definitely take another look.
Moxie has proven himself to be more than capable of building such a system, but the author of SureSpot seems more than competent too. See the section titled "Technical Overview" on:
Interesting fact: TextSecure wasn't made open source until it was bought by Twitter: https://dev.twitter.com/blog/whispers-are-true - IIRC, prior to this the website claimed it was open source, but offered no way of getting the source, and if you asked for it, you would find out it was only given to trusted third parties to perform security reviews.
Moxie has proven himself to be more than capable of building such a system, but the author of SureSpot seems more than competent too. See the section titled "Technical Overview" on:
https://www.surespot.me/documents/how_surespot_works.html
Interesting fact: TextSecure wasn't made open source until it was bought by Twitter: https://dev.twitter.com/blog/whispers-are-true - IIRC, prior to this the website claimed it was open source, but offered no way of getting the source, and if you asked for it, you would find out it was only given to trusted third parties to perform security reviews.