This is a fairly popular topic so do a search for further info.

The 6 cyl also has an aluminium head susceptible to overheating, other then that its apparently quite durable.

Aside from the power, the 6 is also quieter and smoother.

I am faced with the same choice and I think I will go for the iron-head 5 cyl, If i had a bit more $$ to spend, I think id get the SDL.