other side of leather over vinyl (discounting drying out) is it getting wet. when I had a leather saddle on my bike it came with a little cover. I know leather jackets get wet but they are not possibly left outside during rain for a while thus maybe damaging them.
Maybe others on this site who have had leather seats may say otherwise, in which case I stand corrected.
then of course there is cost...
know this varies from place to place but I got my seat & rear seat (not the sissy bar pad) replaced with vinyl for around $50 - it did not have cross pattern like original Savage seat just a "solid" top (matter of cost - cheaper) but I think it looks cool like that (sorry can't take photos & upload - don't have experience w/camera to do it)
just my 2 cents, for what they are worth