Leather... and treating it is not a real big deal... leather breathes.... leather will fit you, it stretches to conform to you. Vinyl gets hot... it sweats... it gets hot... and so on... 
When you buy a nice luxury car, does it come with leather seats or vinyl? 
I totally agree with the conforming, breathing, comfort, etc of leather. But the leather in my nice luxury car never gets rained on or soapy wet when I wash the vehicle and never sees much sun that hasn't already gone through UV filtering glass.
If you take care of it, "outside" leather can work out just fine (especially if garage kept... damn I wish I had room in the garage for my Concours). My leather riding boots aren't looking so hot anymore (since they are often outside and have been wet almost as often as the bike), but I admit I haven't taken much care of them
Of course, I take care of my "inside" car leather.... Lexol maybe twice a year (leather needs to maintain conditioners to prevent it from being damaged). Hmm... maybe I would possibly consider a leather motorcycle seat, I would certainly be willing to treat it and take care of it.
Damnit, OK, I will at least drag out my boots this weekend and clean and seal them.
