it's kind of like the 'best hard drive for car use' debate. while the physics and theories easily say that 2.5" drives are more durable because of less moving mass, but reality has not shown the same thing. guys have run 3.5" drives through car wrecks with zero issues, and others can't get a 2.5" drive to last a year.
the concept behind mounting it blowing into the trunk from the cabin is that it will push cool air into the trunk. i think it is like trying to drain the ocean with a bucket.
same for blowing the hot air of the trunk into the cabin.
neither does anything for directly cooling the hardware, just making cooler air available to the hardware.. like making you run 5 miles, and putting a table of nice cool water a mile away from the finish line. sure you'll get to it, but you'll be a whole lot happier if i bring the cool water to you at the finish line..
now, in really hot areas, it can be a good option to do, but i don't think it should be the only option-- it helps give cool air to the hardware, but doesn't completely fix the issue of cooling the hardware..
basically, if i had to choose only one of the 3 options, pushing air across the hardware makes the most sense to me, as any hot hardware can be cooled slightly by air moving across it.
but a combo of all 3 options is really the best case scenario.
that is, one fan to push cool air into the trunk, with a fan or ducting to push that cool air across the hardware, with either another fan or more ducting to push the air back into the cabin.
just like carpc's, there is no cut-and-dried answer