Exotel is both powered on top of Twilio OpenVBX (Open Source Cloud Platform on top of Twilio's core messaging infrastructure).
In terms of technical integration complexity both Kookoo & Exotel have similar looking API's, having a REST endpoint which can be invoked for placing calls/sending out SMS'es.
Twilio at this moment only supports promotional SMS. This means that these SMS'es cannot be delivered between 9pm and 9am as per TRAI regulations.