Which one to use for process monitoring and why?
Both solutions are good, and there are some pros and cons for both of them.
God config file is written in Ruby, so you can do basically everything Ruby allows you to do, and it's a lot of stuff. Monit has to be configured using its own syntax, it's usually OK but more restrictive. Anyway, you can also generate monit config with Ruby (as a part of your deployment strategy).
Also, monit uses less resources, so if you're on VPS or just don't have any spare memory, monit could be a better choice. Personally, I prefer god, as it's more configurable.
Here's a very good screencast on god. There's also a lot of feedback in comments to this screencast.