Since I've switched to sumo from conventional pulling my traps have had a definite increase in growth.
The athletes performed four sets using their 12-rep max on both the sumo- and conventional-style deadlift while the scientists recorded muscle activity in the middle quads (rectus femoris), outer quads (vastus lateralis), inner quads (vastus medialis), outer hamstrings (biceps femoris), inner hamstrings (semitendinosus/semimembranosus), glutes, inner things (adductors), calves, abs, external obliques, and middle traps and upper traps. They found that the sumo-style deadlift increased the use of the inner and outer quads, inner hamstrings, and upper traps muscles more than the conventional deadlift.
THE VERDICT The major difference between the two deadlifts appears to be that the sumo-style deadlift utilizes more of the quads, the inner hams and upper traps.
Here's an interesting article discussing the differences in what muscles are being used between the two.