Currently, Macports has many more packages (~18.6 K) than there are Homebrew formulae (~3.1K), owing to its maturity. Homebrew is slowly catching up though.
Macport packages tend to be maintained by a single person.
Macports can keep multiple versions of packages around, and you can enable or disable them to test things out. Sometimes this list can get corrupted and you have to manually edit it to get things back in order, although this is not too hard.
Both package managers will ask to be regularly updated. This can take some time.
Note: you can have both package managers on your system! It is not one or the other. Brew might complain but Macports won't.
Also, if you are dealing with python or ruby packages, use a virtual environment wherever possible.