I dunno.. Isn't climbing and sprinting more about the engine? Ride quality and subsequently fatique at the end of your 20, 50 100 miles might be more of what it all comes down to.. Unless your whole ride is a climb or sprint! (go light & stiff!!)
Is there such a difference that a week or so of training couldn't make up for??
3 pounds might be truly a worthwile difference.. If its shaved off the time of race day!! I think the different materials may be better suited to a certain style of rider, or a certain type of riding. Personally (and this is coming from someone who has only had 4 bikes in my lifetime) 20 inch BMX steel, heavy ass CROMO 26 inch Univega MTB 15 yrs ago (suspension? ha!), cannondale killer V (old massive cannondale aluminum, NO suspension) and now lemond poprad reynolds 853 cross bike.... I think you adapt to whatever you ride pretty quickly, and unless you make a bad choice for you (which is. . I have no Idea) or make a bad choice for how you ride, you wont really notice because unless youre a top top top rider youll adapt and get pretty good at whatever you have. (yes Id love to have a bike of each material to actually side by side and ride depending on how Im feeling... but this wont be for a while ; )
Thylacine's #3 comment above is pretty straight I think.. honestly when we run 100 lbs plus in our tires, how much softness can we expect.. get a little gel under your butt for that.
Actually being happy and proud of what you ride is what its all about. if you're a techno geek with a hint of nostalgia you may love steel w/carbon seatstays, if you're a bottom line weight weenie youll love new disposable aluminum, a rich(er) steel lover may go for a TI gem. . . if you like red youll love your red frame whatever it is!!! and methinks that if youre in love with your bicycle then no matter what it is youll ride it your best!! no frame can be scientifically proven to give you 3 MPH average gain over another for everyone!! (unles there's a really drastic weight reduction.. but as mentioned above.. rotating mass is the best mass to reduce (wheels, pedals shoes tires tubes)
my heartfelt 2 cents
~Steve
(materials science is my baby, there is beauty in ALL quality frames, get one of each and ride what matches (in your head) how you're feeling that day!)