I did manage to do a side by side test the other day and the LS lens just seems to me to be fractionally clearer at 100% crop factor and has the added benefit of not shaking the camera like the focal plane versions do (even with Mirror up).
Our tests agreed with both: modestly sharper (most notable at wider apertures on the widest and longest zoom settings) and less camera shake between 1/8th and 1/30th. I did not notice any significant difference once stopped down to f/11.
Also has a focal-length lock and a nicer feeling construction (albeit at the cost of being larger and heavier, and also modestly more front-balanced in weight).
And of course it syncs with flash at high speeds rather than the 1/125th of the D.
It's borderline whether I'd suggest someone with a 75-150D lens to upgrade to a 75-150LS. It's easier to recommend someone buying for the first time to buy the LS over the D. I guess it would depend mostly on how often you shoot in the 1/8th to 1/30th range and how often you shoot near wide open, and how much weight matters to you.
You should be able to rent the lens for a meaningful period (3-5 days) and count the rental towards purchase if you decide to - at least that's our policy. That would be the best (and probably only) way to determine if it's worth it for you.