Nrmally I choose the cheapest unit to re-inforce.
Since I am an Axis fanboy, it's either the Pios or if I'm playing OKW, volks
Pios are an excellent choice because they have extended sight range which is always ideal with HMGs. It gives them more of a chance to suppress approaching squads before they get too close.
I also recommend thinking about sometimes using sturmpioneers instead of volks to recrew important HMGs that are near the frontlines. That way they can actually hold their own against flanking squads thanks to their STGs. It probably does chew up more population though, and is slower to reinforce. It's so satisfying when you force off a squad without re-aiming your MG though.