my 3WT 7'6" Winston is my everyday rod for exactly the reasons you mention: lots of small water fishing in very treesy streams. i put a DT line on it on the advice of my fly shop. when the line got beat up after a couple years from so much successful fishing, i went back and the shop reversed the line. wow! new line feel and action, for free!

it's my understanding that the actual weight of the lines is most of the difference between them. heavier line=easier casting seems to be one theory. if you're after fish close up, you're not going to want to go too heavy, nor do you want to overweight your light rod and ignore the reasons for getting it in the first place.

if the fly shop has a way for you to try the different lines on the new rod, maybe on some sample reels, take the time because they do feel different. the shop folk may also know anecdotally which line takes most advantage of the characteristics of the rod.