I'm probably not qualified to answer as I don't own either, but since no one else has jumped in for the last six hours, here you go...
The major difference is the Vapor King is a two piece unit that uses 'cartomizers', whereas the 901 is a three piece unit consisting of a battery, atomizer, and cartridge.
While the cartomizers supposedly last longer, they are meant to be disposable. People do refill them, but from what little I have read it's a bit more complicated. I also understand that the two piece cartomized ecigs are more of a 'specialty' item and aren't carried by as many vendors.
The 901 is everywhere, easy to maintain, and you can very easily (and cheaply) get replacement parts for. They are more 'standardized', and you can mix and match components from various vendors. You also have more options that you could custom tailor to suit your needs, such as high or low bridges.
Lastly, there are 901 adapters for various other PVs out there, which will allow you to use your 901 atomizers and carts on other devices. This is a big plus in my opinion, as you can make an investment now in backup/replacement parts that can be used later down the road when/if you move to other devices. Like most people, I started out just having to have something that looked like a cigarette. I quickly got over that and have moved on to the Janty Stick as my primary device which looks nothing like a cigarette. Now I'm looking at moving up to a Prodigy or Chuck for at home power vaping.
There is a ton of info here on both of these models, and I don't think you could really go wrong with either in terms of usability and performance. But there is definitely pros and cons to each, in my opinion. Good luck!