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.
Twilio isn't indefinitely free for developing like Tropo, but they do give you $30 credit when you sign up, which goes a long way towards pre-production testing. Twilio can do international outbound calls, but as you say they don't yet have international inbound.
Twilio and Tropo both have a REST API. The main difference here is that you reply to Twilio with XML and Tropo uses JSON. Although proprietary, either one is easier to use than VoiceXML IMO.
Tropo has an amazing number of nice voices you can use to read off text to the caller. Twilio has just "male" and "female", last time I checked. On the other hand, correct me if I'm wrong, but I don't see where Tropo could play an MP3 to the caller, whereas there are lots of ways to play recorded music or messages to the caller with Twilio. As far as recording, you have to give Tropo a place to send recordings, whereas Twilio will store the recordings for you. Twilio also has tons of API methods to access data, like list all your recordings, call logs, and so on. I don't see that for Tropo.
Hmm, this is getting long. I think a comparison blog post is in order. I love the fact that there are at least two smart, highly motivated companies in this space. I hope Twilio and Tropo push each other to greatness.
And you can store the recordings on Tropo -- your Tropo app space has FTP and WebDAV credentials, so you can tell your app to FTP it to Tropo's webhosting. We don't want to store your recording forever, but fell free to use it for short-term recordings.
Could you please answer Barnabas to clarify his miss understanding, because Tropo has great voices.