There are three things I prefer about KSP over orbiter:
1. Building ships. You can do it to a certain extent if you know your way with some coding, but let's face it, it is a lot easier whit the lego-style KSP.
2. The scenery. When you land somewhere in KSP, you always have an awesome view. The terrain is nice and there is a lot to visit.
3. The carreer mode. I like orbiter, but I don't feel like I have a goal. I like to make some missions to ISS using various craft or sending a mission to mars, but that's about it. I would like to build my own station, but using the notepad to put the module in the shuttle or on the rocket is usually more a pain in the ass than anything else (at least for me), so I just open KSP and assemble the ship and launch.
On the other hand though, there are also things I prefer for Orbiter:
1. Realism. By that I mean two things. First of all, it is our own solar system. KSP is a "copy" of our solar system, but it is not quite the same. Also, it is scaled down. Second, physics realism. Especially when it comes to that all mighty aerobrake. I won't hide it that when I see someone coming to Duna at a ridiculously high velocity and aerobraking 3 km over the surface with a ship that looks like the ISS and coming out whitout a scratch, that feels a bit weird.
2. All-made rockets. I know I said I liike KSP because I can build my own rockets, but sometime it can get annoying. You want to put a sattellite around Duna? Let's build something that has at least 40 boosters otherwise you won't get you payload to orbit. The ships are less nice from a look point of view and (I know it is still beta but) sometimes parts brake for no reasons and everything explode. I like having my delta IV Heavy and just puting my sat on top of it and launching it to mars. It looks nice and I don't need a 9 boosters aspergus staging rocket that looks bad to send it where I want it.
That being said, the debate if one or the other is better is pointless. One is a simulator built by one man alone (with some help from the community) and the other one is a game devlopped by a team of guys doing this as thier job.
For the scenery part though after seeing the image of Orbiter with some terrain, it might change my views of things.