SASS for sure. I've used sass in development for years now, and I prefer to not use LESS. Having LESS as a requirement was an immediate read flag for me :)
[1] http://blog.danieljost.com/scss-on-a-remote-server/ [2] https://github.com/dahlia/libsass-python
You might want to look into the C implementation? https://github.com/hcatlin/libsass