I'm a member of the TextMarks team, for what it's worth. My perspective, like many participants in this discussion, is that the SMS space is very crowded. That said, if you're looking for a good API--which is what Twilio is all about--- the competition is actually pretty small. Plivo is good. Trumpia and Eztexting are okay too, but probably aren't as stable, fast, and well supported as TextMarks, Twilio and Plivo. That's kind of the bottom line, in my view. Best of luck with your project.