My wife and I have had sex a couple thousand times (no joking, it's been 20 years and we have done it a LOT) and even now, still after all this time, she finds that my girth is a bit much!! And I'm like 5.25"!!! I know it's not the girthiest, nowhere near 6" like some men here, but if I pump I get up to 5.5" and it's just way too much for her. She can't fuck again for at least a day maybe two. However, it seems like girth is the factor for stimulating the vagina and clitoris during intercourse. It's been lots of surveys and testimonials from women in this regard: girth over length.
However, I'm much longer than girthy, and if I weren't long enough I wouldn't be able to get into the extreme far back of my wife's vagina. There's a little pocket, it's extremely tight like a ring that my glans "pops" through, and if I was even just 1/2 an inch shorter I don't think I'd get in there. When I can get in (it's not every time) she absolutely loses it! It creates an even closer bond for us, like even more of me is in her and we both feel it at the same time very distinctly, so it's highly and heavily emotional. Length over girth for that!
Also, while maybe not the most pleasurable feeling, if you have more length you can get more "up in them guts" so to speak. On prolific nights, especially if the angles are right (or wrong lol), my wife says "man, you beat me up inside! My organs feel like they got bruised!!" I feel a little bad but also quite proud at the fact that I'm able to "beat up" her insides beyond just the pussy!! Sometimes she's bent over a little for a few minutes holding herself. I tell her I don't have to go so deep so vigorously, but she likes it, so...
My vote is both are extremely important, but it all comes down to the female's preference and what gets her off physically, and, emotionally.