As a matter of fact I JUST got home from being at the Monster 1200 training in Las Vegas where we got to do a 2-hour ride on it! The national launch of the Monster 1200 is coming up next Saturday April 5th, when it will debut in dealerships all over North America. I think in general most people would not be debating between a Monster and an 899, they are vastly different bikes with a completely different focus. One is a full-on sport bike and the other is a naked street bike. Monster is WAY more comfortable, versatile, easier to ride, lower seat height, etc. 899 is higher performance, sporty riding position with tall seat height, razor-sharp handling, etc. Despite specs saying the HP is nearly the same, the 899 revs up a lot quicker and has a more aggressive power delivery, and it's a lot lighter, it's a noticeably faster bike. There's a big difference between the race-tuned Superquadro engine in the 899 and the street-tuned Testastretta 11 Degree engine in the Monster.
One side note though, since you mentioned it, the Monster 1200 puts off a lot of heat from the engine, just like the 899 and pretty much every current liquid-cooled Ducati. When we were sitting at stop lights on our ride, I noticed the heat quite a bit, once you're moving it's irrelevant. So I would say heat would be no concern at all in picking one bike over the other, they both put off heat.