Do any of you switch between Perpetuem and Sustained Energy for longer rides? If so, what makes you choose one or the other, and do you interchange them?

I'm just getting into longer distance riding; I completed my first century a couple of weeks back. I think my fuelling strategy worked out well - I drank 3bottles of perpetuem and 4 bottles of HEED during my ride. When on HEED I popped an extra gel (i.e. ~200cal total) and a couple of endurolytes. Finished my century feeling fresher than I could have hoped (6h 37m; 103miles, 6000ft climbing, ~200cal/hr). The perpetuem really helped, but I don't think I could have used that exclusively - the temperature got to 90F later in the ride and it was just a bit heavy for me by then.