Personally, I would get the 6.03 over the 6.05, simply because the 6.03 is solid steel, while the 6.05 is coated. Now, it's an EXTREMELY strong coating, but if you ever want to extend the hop up window, you'll have problems.
And the thing about barrel length is a little complicated. The idea is that after 450mmish of barrel the bb has completely stabilized and needs to leave the barrel immediately to prevent any possibilities of off axis spin being applied to the bb from dirt or imperfections in the system. But at the same time, you need enough air to push the bb all the way down the barrel. Again, ideally, the air will stop accelerating the moment your bb leaves the barrel, lessening the air POP, and removing the possibility of air turbulence messing up your bbs trajectory if your system is extremely over volumed. And after that, you still need to consider the bb that you are using, different sizes, and weights allow more or less air to escape around the bb, meaning you'll need more air for heavier/smaller bbs, and less air for lighter/larger bbs.
But then you start getting into the argument of how can 450mm be the point that all bbs stabilize in a barrel no matter the set up, whether it be a 320fps CQB or a 700fps plinker. Using different weight bbs, and different sizes should change that number even more than just the simple 450mm barrel. I don't know for sure, it's mostly theory crafting without much experimental facts.
From my experience, barrel length doesn't matter all that much, it's the quality of the barrel that effects the accuracy of the shot.