If you're doing something very simple (installing a pure-Perl module or set of modules without configuration), either one works fine. If you need to do something more complex, Module::Build may be easier to understand -- it's conceptually simpler, with fewer cross-platform moving parts.

I use Module::Build exclusively because I think ExtUtils::MakeMaker is a big ball of horrible hacks, and I want it to go away forever. Not everyone agrees.