RAH wins hands down on playability. Wow, I couldn't disagree more. Maybe because the 25th style figure you used for comparison is quite possible the worst example you could have chosen. Wraith is horrible when it comes to articulation and movement. He's severely limited due to the gauntlets, wires and armor. You could have chosen just about any other figure and found he's lacking. That would be like choosing a star brigade armor tech figure as the RAH representative.
If you want to know why I think the 25th is slightly better than the other incarnations, pick up just about any Snake Eyes or Storm Shadow. The range of motion for those figures make it so you can put them in all kinds of poses. I've sat there and had those two battle it out. They were able to do stuff I could only imagine RAH and New Sculpt figures doing. I guess I never played one handed. I like to pose one figure. Have him hold that pose while I have the other guy perform an action. Maybe that's just me and we're talking about different play styles rather than figures.
As far as customizing, I tend to agree with Nova and Lance. While an exacto might cut better through the softer plasticy parts, the finish product is never as clean as RAH. I can make those old figures look factory after a modification. Then I will apply paint and mess it all up.

With the new sculpt and 25th, if I don't do a modification at a certain point on the figure (torso meets the waist, gloves meet the forearm, etc) than any modifications are pretty obvious. I have to cover up any of the changes I make.