I've had both, and much prefer the rear discharge for a couple of reasons. First, with all the other chores on my plate the mowing was generally done when the grass was a bit long and sometimes wet from a heavy dew. The side discharge didn't like that, and as the chute became progressively narrower the mowing slowed down considerably. Secondly, it also blew the clippings into a neat windrow on the unmowed grass, so the mower would have to labor to remow those big clumps on the next pass.
I haven't mowed since the wife got sheep! /forums/images/graemlins/grin.gif
Pete