Well, the stroke of a spring rifle would not have an effect, as it is a manual cocking system, and we don't recycle our bolts before the bb leaves the barrel, so length really wouldn't have an effect.
Also, I know Oakey (Shoots a stock VSR barrel, I think it's a tightbore though) and I've seen him get out to 90 yards easily, I mean.. I'm sure their could have been some lob, or a good hop up, but reguardless, he hit it.
It was from that shot that I made the assumption to believe that barrel length will not significantly effect performance and/or accuracy, so bare that in mind.
Now, about the diameter. With a 6.01 you will see more power, that is certain, a tighter bored barrel makes for a tighter seal around the bb, making more pressure and more power, that's basic physics that I think anyone here can agree on, whether it be 5 or 10 FPS more than a 6.03, it'll be there.
Secondly, you have to know that with a 6.01, if you get even one grain of sand in your barrel, chances are it's bye bye for the internal portion of it, because that one grain of sand will catch onto a bb and it would be like taking a car key to the inside of it, if you could imagine that.
Now, with a 6.03, you'll notice that you can have a pretty gunky barrel and still shoot, knowing that no harm will come to it, because the bb can fit around that grain of sand, and continue on it's journey, of course.. Accuracy would still be bad, but at least you'd know that your barrel was good.
My advice is if you can't keep a clean barrel, go with the 6.03, but if you really want tight grouping and a little extra sass, go with the 6.01 . Of cource, the grouping difference will be at most.. 3 inches, but still.. It's there.