I asked this same question about a month ago and the general consensus on the forum was that the Mustang was the way to go unless you get some other true custom seat like Day Long. Mustang is definitely the cheapest of the two.
Also, the Kawasaki gel seat is exactly the same as the stock seat except they put a small square of gel right in the center of the seat. The gel is only about the size of a CD case. One of the other Vulcan brothers on the forum actually sent me a picture of his K-gel seat with the cover off. He fixed it by taking a piece of raw gel like you can buy at Saddlemen and then just stapled the cover back over. That at least gave him full gel coverage.
You might be able to find the old thread with the pics if you search for threads by Jeep Pirate. I can't remember the title, but it was something like "Which seat?".
'07 VN900 Classic
Horsepower is an illusion.
Torque is the source of all good things.