I had a Spesh Bighit (size M, mini-DH esque) and now have a proper DH bike. The littler bike was easier to jump on and ride, took no mental adjustment to get used to it. However the big DH bike (size L so even more biggerer) took a while to get used to, needs manhandling and more effort, but you can go bigger/ faster once you understand the angles / wheelbase.
IME I dont think the suspension travel will be the main factor, its the combination of angles of the dangles, wheelbase, suspension progression and stiffosity.
I love my DH bike and wouldnt want to change it now, whereas my mate just sold his DH and bought a freeridey type bike and loves it.
edit- just buy the one which looks best.