Solidly in the "choice C" category. If I'm playing against someone who "has" to run unique or custom "munchkin" 'Mechs (some people won't play "stock"), then I'll sometimes follow suit and field my own "munchkin" design in opposition. In almost any campaign situation, I'm going to choose a standard variant, usually with some particular House focus. There are times where the campaign calls for some "unusual" opponent, and that may involve a unique or custom 'Mech, but generally there's some standard variant that will provide the same level of "Uh, oh!" drama that the encounter demands.
I'd much rather play with a variety of assorted "stuff" and figure out how to make the best use of it, rather than have all of the goodies from the start.