Tropo is 100% free for developers. No credits, no limits on minutes, no ads played to you or your callers. You even get your own phone number. You decide when you're ready for production and when you're ready to start paying.
Once you're in production, Tropo's SMS and toll free minutes are cheaper. Transcription is free with Tropo. Twilio's phone numbers are cheaper by a couple of dollars each. If you're going to have less than 150 minutes per month of usage or less than 200 text messages in a month, you'll save about a buck a month using Twilio.
Tropo has speech recognition. Instead of just "push 1 for this, push 2 for that" your callers can talk to your application.
Tropo is international. We have numbers in 30 countries, speech recognition and text to speech in 8 languages, and can send SMS worldwide. More languages and countries are coming.
Tropo is a unified API. No need for one app for voice, another for SMS and a third for conferencing. A single application can do all of that. The code you use to say something over the phone can also respond via SMS, IM, and Twitter.
Tropo is more than just phones. As mentioned above, we do all the major IM networks and Twitter too. No need to code to different APIs. Click a button and you have a Jabber bot. Or a Twitter bot. Or AIM. You get the idea.
Tropo's SMS supports short codes. If you want a shortcode for your application, let us know and we'll help you out with that.
Tropo offers a web services model where we post information about a call to your server and you reply with what you want us to do with the call. We also offer a scripting model where we've extended popular web scripting languages with built-in functions for communications. You run your code on our server for reduced latency.
Twilio can do some nifty things while you're in a call. They give you some REST apis you can hit to control sessions in progress. Voxeo's enterprise products have offered that for years, but it hasn't made it into Tropo yet.
Tropo lets you call your app via SIP and Skype. As mentioned elsewhere in this thread, that allows you to buy a number from any SIP provider and connect it to your application. It also allows you to forward your SIP PBX over to your application under certain conditions. Maybe you have an Asterisk box running your business. During off hours, forward to Tropo so your application can answer calls and give information to your callers.
Twilio lets you provision new numbers programatically. Tropo doesn't do that yet, but it's coming soon.
Tropo is built and hosted by telephony experts using the same platforms that power some of the largest companies in the world. Tropo is a product of Voxeo, a company that's been building voice platforms for over 10 years. We have data centers around the world that are optimized for voice. Your voice application isn't sharing space on an EC2 instance with somebody's web server.
Twilio's documentation is better. We're working on it, and in the meantime, our 24x7 support will answer your questions quickly. In most cases in less than an hour.
I'm sure that the Twilio folks have some additional thoughts on how they're different than Tropo. Instead of speaking for them too much, I'll let them reply.