In addition to our first criteria--is the price right?--there are a few more worth considering.
For background on mobile giving, MobileActive gets a big gold star from me. They're the NTEN of mobile for good, basically. This article is a fantastic snapshot of the state of mobile giving: http://mobileactive.org/
One of the things I'd look for in a mobile giving provider is longevity. These campaigns will build over time, I'd want to start with a provider that I thought would be around for a while. Criteria #2: Will the mobile giving ASP (application service provider) be around for at least two years? Not unlike choosing a CRM, you don't want to keep changing horses mid race.
I recommend starting with the Mobile Giving Foundation's recommendations.
They certify both ASPs and mobile agencies. They have standards for which nonprofits can participate. Alternately, the mGive Foundation is a parallel operator. Here are their standards for nonprofits. MGive only works with one ASP. You may have heard of mGive recently because it is the provider to the Red Cross.
I'd be wary of hiring a non-approved ASP. It's a bit of a wild west right now for mobile giving ASPs, and it's important to take into account the credibility of your ASP in the minds of donors and the general public. Criteria #3: Does the mobile giving ASP appear trustworthy to your donors? Have they run credible campaigns? Are their fees out of line? Note that the promotional emails you've sent may actually be resellers for approved ASPS--it's a good thing to ask any attractive offer who their mobile giving ASP is.
I'm looking for a resource that details all of the mobile giving options for non-Red Cross size orgs. I haven't found it for you yet. I may have to write it. Save me from writing is by suggesting one in the comments, please.