Vice has stated in the past he uses a mixture of both.. I don't know if its a custom hackup, or just Dark GDK. But its been a while since I read that thread, so I dunno for sure.
I know he only recently (past few years) starting trying to port things from DB Classic to Pro (apparently back then Pro had some major issues and he thought Classic was more professional) so I'm assuming Mercs was done in DBC or DBPro..
Mercs is the best looking game to date, by far.. especially as far as planetary terrain is concerned.
Mercs was also featured in a TGC Newsletter
http://www.thegamecreators.com/pages/newsletters/newsletter_issue_90.html#10