In my opinion, the shorter ones look nicer, but what matters is whether or not they derail your cars. Testing on your layout is the most important thing. If you run your trains and it doesn't derail, keep the short. If not, the long one's work. I have found with mine, it depends on the engine. I have two SD70MAC's; neither derail cars from the rear coupler, but one of them derails cars from the front coupler, so I have a long one there. I also have one SD70m, and it normally pulls really long cars (automax's) and it would derail the cars on one switch, so I put the long one one and no more derailments.
Again, testing on your layout is the most important thing.