I've decided to learn D, and I'm wondering which standard library I should use. Should I use Phobos or Tango? What are the pros and cons of each?
Tango. It's more object-oriented where appropriate, it includes containers (like STL or Java Collections), it's got an active development team, it has more momentum (it may soon be incorporated into the official compiler), and it's got real documentation, including Learn to Tango with D.
It looks like Tango may soon be incorporated into Walter's releases.
I've had little experience with both (kinda ..)
Phobos is more flat and python-like, but quite incomplete.
Tango is more Java-like, it makes simple things complicated.
I personally prefer to go with phobos, unless you need a library that depends on Tango (such as DWT).
If you need to use D2 then phobos is what you should use for now but tango for D2 is in development.
tangobos allows to use tango and phobos together at the same time. In D2 both work together anyway as they both make use of the separate druntime.