Fuelpak. I bought one for my 2008 FXDC. Last week I installed a V&H Big Radius and Screaming Eagle intake. Holy cow it was easy to use. Once I got it plugged in, I was ready to ride in 5 minutes.
The dealer download will cost more, and you can't modify it later. The V&H Fuelpak was the easiest part of the upgrade.
I know alot of folks on here are pushing the Dobeck tuner, and I can't say it's any better or worse, because I never used one. All I know was that the V&H Fuelpak cost $215 and was a piece of cake to use.