Doctrine is considered the most popular PHP ORM out there. As such, it has a big community which brings a lot of perks. Namely a great deal of learning resources and it's easier to find help for any issue you might have.
Doctrine implements its own object-oriented SQL dialect called DQL. It's inspired by Hibernate's HQL and it provides developers with a powerful alternative which helps in gaining flexibility without having to resort to code duplication.
Since it's a fairly large library, it would be a bit of an overkill to be used in a small project. Since for example it wouldn't really make much sense for the ORM to take up to half of the project's size.
Propel uses Active Record, which is a rather simple to understand implementation of an ORM. It's also the most widely adopted implementation. In simple words: an object represents a row, you can create or edit an object to create or edit the corresponding row.