I also use voip.ms. Unfortunately their API and features have stagnated for quite a while. Even a basic feature like the ability to upload a voicemail greeting has not been implemented yet.
That being said the features they do have implemented work well and the pricing is attractive.
I can't really tell what Plivo offers other than an API to build custom solutions.
I use it as well.
It does seem to have slowed in terms of new features.. but it's the absolutely no-hassle, reliable, functional service that I just love. It's cheap, it does what it does properly, you don't get spammed or tricked into anything whatsoever, and if you do happen to have a billing issue or something, their support has always come through over email with no problems at all for me.
They have their act together.