Whoa... I'm gonna come in here and just offer my own little opinion/advice...
I am 6'2" 175 lbs and I ride a 153 Jibstick (08/09, but it's 100% unchanged other than graphic for 2010 (and 2011 too)).
It is about a 5 stiffness compared to an Artifact's maybe 3 stiffness. I would say the K2 WWW is like 1.5 or 2--- it's real weird feeling, I hate that board.
I ended up buying a 152 Nitro Sub Zero for rails because the flex is like an Artifact and I got a killer price on it. And I was really impressed by it.
Lots of dudes my size ride something about 152 for rails (Ben Bilocq, Jon Kooley) so I wouldn't listen to any of these cats that say you should get a 156. That's nuts.
For you, for a rail only board, get a 145, or 147/8. Nothing bigger. If you are set on getting a rocker board I would suggest looking at an Artifact 1985, a Sub Zero, a Swindle (flat camber one), or maybe a Horrorscope FK or Indoor survival FK.
If you are having trouble with presses on an Artifact, a Jibstick honestly isn't going to be the best thing for you. If you want a Stepchild (I love their boards) check out a 2011 Salaryman or Maiden board, as those have a softer Artifact-like press and also will come in Rocker. Jibstick is one of my favorite boards, but I find myself only needing it's pop and stiffer flex for longer features. For normal handrails or the like, a softer board will be better for learning (i'm not a pro). The Jibstick takes some real wild commitment, but it's worth it if you have the balls. If you find a 148 Jibstick, that might be pretty cool.
Good luck. Unfortunately finding the right board is going out and trying different shit and finding what your riding syle likes. Sometimes that means buying the wrong board. Just roll with it, and try to demo or try friends' boards as much as possible.