Should I use thin or unicorn on Heroku Cedar

No reason not to do it - I use Unicorn on Heroku with much success.

Update -- 3 months later.

I have been using unicorn in production for 3 months, and I have been very pleased. I use 4 unicorn workers per dyno.

One thing you do need to keep an eye out for is memory consumption and leakage. In effect instead of having 512MB of memory per dyno -- you have that divided by the number of heroku workers. But keeping that in mind -- it has been a great cost saver