I've had both
The Ultimate cost more but was harder, I was disappointed in it. The Mustang is superior, I have used one on several bikes. Get the seat with the driver backrest and the matching pillion for your passenger if you have one. You won't regret it, except of course when you pay for it. But hey, we all spend $10,000 - $30,000 for bikes but whine to spend $600 for a good seat which will make it so you might actually ENJOY your ride instead of TOUGHING it out. You'll win kudos from the back seat rider too!
Also, like the other guy said. Get some seat beads (beadrider.com) and the combo of the 2 will make you smile! These 2 things are in the top 10 of what anyone can do to increase their riding experience!