Since it's easier to be disarmed of a firearm than a knife (you can grab the firearm with less risk of personal injury than you can a knife or sword), I'd go with whichever weapon I was most familiar with (knives, actually), unless I knew that it would start with some distance between us (at least six feet). In that case, I know I could snap off a semi-accurate shot before my opponent is on top of me.
I also suggested, earlier on somewhere, that if you want to house rule Parrying pistols used in melee but don't think it should be as easy, that you also house rule the Fast quality from the Inquisitor's Handbook onto all pistols. It doesn't make the weapon "fast" as in what we think of - it imposes a -20 penalty to any attempt to Parry the weapon.
Also, a pistol in a melee fight a knife or short sword is in fact more comperable to a pistol than to a whip, flail, quarterstaff, or chainsaw, and it still astonishes me that you draw the line where you do. I mean, feel free to disagree, but in addition to the general involvement of "unrealism" in the game I am really not seeing the "realism" that you're invoking.
A Chainsword would be effectively spraying burning liquid when he starts it up. He could even light allies on fire *gasp* hehe. Anyway yeah that's cool and it's totally happening in my game sometime.