I have both as well and they both serve their purposes. I only got the vapor king to test out as I figured I'd buy them for my parents so they can try and quit as the VK is very low maintenance. I was surprised at how much I like it though.
It really depends on what you want from your PV. Here is a little breakdown although this is just my opinion:
Battery life:
510 - Crap... Couple hrs of straight vaping, tops
VK - Decent 5-6 hrs. I only say decent as the fact it's not producing as much vapor helps the batt time. Definately better than the 510 though.
Juice:
510 - Not locked into PG or VG as you are getting your juice elsewhere.
VK - Cartomizers only come in PG. I hear they are adding some VG flavors soon. You can also refill the cartomizer with VG if you want but I also heard that VG doesn't work well in the carts
Vapor:
510 - Great vapor using manual battery
VK - OK vapor, nothing to write home about but this is an automatic battery. Pretty good vapor for an automatic. With VG juices it may improve some but I don't see it producing more than a 510 even with VG juice. The vapor is enough though and with the VK passthrough it's up there witht he 510.
Throat hit:
510 - Decent.... gets better with each atty over time as gunk builds up.
VK - Superb! Even with little vapor you get a very nice TH.
Ease of use:
510 - With all PVs not using cartomizers you will be tinkering with leaky filler/carts, cleaning and replacing dead attys. I actually enjoy this though but it's not for everyone
VK - Superb again. Grab a cartomizer, screw it on and that's it. This was the only reason i bought one as I'm considering them as gifts to lazy people to help them quit.
Upgradeability:
510 atty - Access to most mods if you ever feel like upping the voltage and producing serious vapage.
VK cartomizer - I don't know if any mods support these things although I guess you could build your own connector for it. I doubt the cartomizer would hold up at high voltage though.
Summary:
510:
I love my 510. I have spent hrs upon hrs tinkering with it getting it to do exactly what I want how I want and I'm very happy with it. If you are not put off by maintenance I would go with a 510 or one of the other favorite starter e-cigs as from there you can move into mods if still not happy. With the higher voltage mods you can create all kinds of vape. A lot of people are put off by the inevitable maintenace though and that's where the VK shines.
VK:
I was surprised with this thing. I thought I wouldn't like it but have no regrets with buying it. As others have said VK customer service is just awesome and you are bound to end up with alot of extras in your purchase. Cartomizers are exactly what they advertise as... no maintenance. The only problems I have with the VK is that it only comes with PG liquid as of now (as stated above they are planning on adding VG flavors)and the vapor isn't as good as my 510.
If you are not put off by maintenance and think there is a chance you will be upgrading to a higher end mod I would go with the 510 or one of the other popular atty models.
If you just want to vape with no hassles and don't mind so-so vapor as long as you get a nice big throat hit, I'd go with the VK.
And they really taste good. Especially the pineapple!!! 
