benmcl, I am personally pro-mac and pro-linux. In general, i tend to dislike the platforme dependant philosophy. But, to be honnest, after playing Half-Life 2, it would be unreasonable to think that any other game engine currently available could be a better "host" for Infilration. That is, of course in my opinion... which is also taking into consideration the modification support for half-life.
When considering an engine, for what ever purposes, you got to take account of all considerations. "Is it platforme dependant?" is one of these consideration. Sadly, nowadays, the answer to this question is "yes" and we have to live with that. On the other hand, I do not thing this is a "critical" consideration in this case, even thought it's a major one. A critical consideration in the case of Infilration would be "does it have network support?" or "does it have modification support".
Taking all things into considerations, my conclusion is that Half-Life 2 is the best choice.
Maybe a good exercise would be to write down every decision considerations in choosing and engine, give them weights (like from 0 to 5 for unimportant to major, with an additional weight for critical considerations).