Your reasoning: One player hasn't shown "he's capable of more than running in a straight line" so another player who just runs in a straight line should be ahead of him.
No he hasn't. That has been Halfpenny by far.
Also sure North was ineffective in November, but he wasn't awful, and Cuthbert wasn't much better. All I remember Cuthbert doing of note was having one run down the touchline and then getting pushed into touch (as he couldn't step inside as he doesn't do anything other than run straight), and also getting embarrassingly (for a man of his 6 foot 6 inch size) smashed backwards by Luke Romano when Romano scored.
The entire Wales backline has been exposed as a bunch of lumps who just run straight, not just North, Cuthbert was largely shut down in November as well.
Also you saying North has been ineffective ever since the World Cup is wrong, he has been ineffective for the last couple of months or so. Ireland for one didn't work him out ... (also note another offload here that Cuthbert rarely ever does). Also he has scored 7 tries this season (third most in the league), which is as many as Cuthbert and not bad for an off form player.
As Olly says, an on form Cuthbert and an off form North are around the same level, and on form North and there is no contest.
Difference. Cuthbert makes a load of errors like were mentioned above, were all in just one season. That was probably the only time Bowe has done that in several seasons.