Others who actually specialize in this movement could probably give you a more complete answer. However, I purchased a "Trap Bar" in the late 90's just for some variety.
I found the bar to be very useful and I agree with you, it seems to put more stress on the glutes and less on the lower back. I also seem to be able to do more weight with the Trap Bar. Although others have seen the opposite results.
I think (unless you are a Powerlifter) you should use both. Variety is the spice of life! I put it in the class of the Leg Press machine, which I also use. It's not as good as doing Squats, but it's a great piece for variety. I think variety is what keeps you training for many years, and perhaps what helps ward off injury.
When I first started deadlifting I used a trap bar. I find it to be easier to hold than a barbell. I hardly ever do trap bar deadlifts anymore, but if you ever want to change up your routine on the deadlift is a good exercise. I have seen a few articles that have refered to them as trap bar squats and used them as an alternative for people who can not squat due to a back injury. You have to do what feels comfortable to you and lift hard.
A lot of people on this board will tell you that the trap bar is for pussies. Screw that.
I really like the trap bar DL. It is much easier to teach than either the conventional or sumo technique. Just like one of the previous posts said, it is a great tool for variety.
I know that the Westside boys talk a lot about deadlifting being really hard on the nervous system, which is why they really limit the frequency of the movement. I have never felt this way when using the trap bar. I think because of the way it moves the center of gravity, it makes the movement a lot less demanding and easier to recover from.
As far as poundages go, I have always found that people use more with the trap bar.
I love Trap Bar Deadlifts. I alternate between them and regular deadlifts. With the trap bar I am able to use my legs more as compared to my lower back with regular deads. I would imagine the trap bar would work great for people with lower body flexibility problems.