I've bought "narrow" and "needle" ferns before, and in my experience "needle" ferns are longer and maybe a little thinner. "Narrow" is more dainty looking, in that it's shorter but still thin in my opinion
From my experiance narrow is less resiliant and is a slower grower, I also find that if pruned frequently needle is much prettier and a much lighter green as well as suiting smaller tanks better, especially it's new leaves which are very thin and very needle like.
It really depends on where you get your narrow leaves fern from. I've bought it from Far east and tropica and both looks similar to a certain extend but the far east variety is a lot longer. I had them in a 18 inches tall tank and all the leaves were growing nearly to the water surface. The width of the leave is about half of that of a philipine or tropica variety fern. The needle leave that I had grows no more than 8" long and it's even more narrow than the narrow leave that i have previously. I've also recently acquired a java fern mini which is even smaller than the needle leaves and you can see the difference in the two leaves quite distinctively if you place them side by side.