* There are still fans of MakeMaker out there, for example Matt S. Trout was defending it in a talk I heard last year. Maybe it is not so legacy as you think
* What is the sense of two installers if everyone has to support both?
* Machines where old perl versions operate usually don't get much new software. But if the maintainers of those machines are allowed to install your modules, why shouldn't they be allowed to also install Module::Build (which "works fine on perl 5.005" according to the docu) ?
So I would recommend going with (b)