lbjgh is right -- you need to spend the $1400 to get all the features of DHP. And yes, you want DHP. It makes a huge difference in the car's handling and the quality of the ride.
I have it on my 35i. I was given a 28i loaner this morning that does not have DHP. By the time I got to work, I was ready to trade the car in! DHP adds a subtle quality to the ride, but it is a quality that is a complete bargain at $1400.
On a side note, the N20 engine in the 28i is very impressive. It doesn't have the off the line grunt that the 35i has, but it provides reasonable torque and works very well.