IMHO You cant have one dimension without the other, i.e., their effects are both the result of an integrated whole effect.
If you use the nerve endings to support an argument then it stands to reason that a longer unit will deliver more girth (pressure) to more nerve endings and nerve endings that are deeper in the vagina thereby creating more overall sensation and pleasure. (some may argue that most of the nerve endings are in the first few inches of the vagina) None the less the woman may feel a lot at her entrance but does not feel the deep sense of vaginal fullness.
On the other side of the coin, it stands to reason that if there is plenty of length (lets say 7+) but no girth then the length itself will no doubt have reaching power deep in the vagina and to all the nerve endings but since the pressure is minimum the nerve endings it reaches will have minimum overall sensation. Pressure is lacking and so the trade off may be that the short fat dick feels better for some women whose nerve endings are primarily at her entrance. Women with more evenly distributed nerve endings through out the whole length of her vagina may opt for the longer thin dick guy despite having less pressure.
With that being said, let us consider that Length and girth is really relative not only to the distributions of nerve endings but also as to the dimensions of the women and what she can accommodate but assuming we are shooting to accommodate the most amount of women (peak of the bell curve) then the options for optimal results are as follows:
For the short fat dick (Lets call him Stumpy):
… Either he becomes longer or he finds a home with a short tight vagina. (Lets call her Jan)
For the long thin dick (Lets call him Slim Jim):
…Either he becomes thicker or he finds a home with a long tight vagina (Lets call her Lisa).
In any case—Since no woman wants a loose vagina and vaginal tightness in a woman is a desired feature by both men and women, it stands to reason that Slim Jim gets to play with both Jan and Lisa, whereby Stumpy only gets to play with Jan.
Therefore I submit to you that length is more important than girth if you had to choose only one dimension.
That and the fact that it is easier to gain girth than it is to gain length. (at least for most pumpers it is)
For the ultimate results I believe that you need to deliver full pressure to the full length of the vagina.