Xbox One does allow more air-flow to it's fan. It allows the console to run quietly and not have any over heating issues, that were too common for 360. Microsoft knew they could not have that same issue again.
About console revisions.... I don't see too many people replacing their larger Xbox One consoles for smaller ones? I've never replaced a console unless it didn't work. They're all going to do the same thing anyway. Play games. Unless you need an extra inch of room, or whatever a slim would give you, or you just don't like the design of the original, why would you care about when a slim model comes out?
If I personally, were interested in an Xbox One, I know the last thing on my mind would be how big the console is. It would go in the same spot my large PS3 is in and my PS3 would go somewhere else.
Speaking from a design standpoint, both systems are fine... Wish PS4 Had more vents, but as long as it works without breaking it's fine. Same goes for Xbox one.
What I don't like about either console is that they both are simply not very good looking. PS4 looks like a discarded piece that fell off Metal Gear Rex, and the Xbox does look like a VCR, or something.
I don't think Metal Gear Rex is a bad design, but you get what I mean. Neither console is perfect, but size is the last thing on my mind. Hell, there are times my PS3 gets stupid-loud so anything different at this point would be fine. It's bothersome, yeah, but it isn't like it's impacting my experience a whole lot.