We use both Twilio and Plivo in our text messaging products in CallHub. Both are great vendors to work with. We'll not move to Amazon just for a lower price. The things that we value are,

1. Quality of delivery

2. A consistent API across all our telephony needs.

3. Great support when we need it

I especially love the fact that these vendors are constantly innovating and give us early access to those innovations as well. The most recent was Twilio's release of studio.