It's kind of difficult to compare. I actually think of submodules as being much closer to svn externals in nature, just very poorly integrated (I'm not sure anyone on the git core team actually uses submodules, in things I've read as justification for its poor implementation from core team members it seems like they only added it as a checkbox feature).

Subtrees are quite different from either. They make the imported repository literally a part of the commit graph of the parent tree through a subtree-merge-commit. It then does some extra work to help you bring in new changes or export new changes back.

I think it's a better approach than either, but it's also not exactly the same thing, basically.