1. Cheaper
2. Multiple voices and accents (UK British Male & Female)
3. Use any number as caller ID.
4. Calls during development are free.
5. Recordings are a breeze
6. Voice recognition from a set of options instead of pressing digits (eg. "Say yes, no or repeat").
On the downside, I've found their system wasn't quite as easy to get going with as Twilio, but it is much more powerful.
Twilio have been more visible and active - Tropo seem slightly behind in evangelism and generally pimping their product.