a quick bit of background:
i have been doing my serious training on a carbon fiber Felt.
i have been doing lunchtime/slow chillin' rides on a steel Masi. it has platforms with toe clips for sneakers.
almost all of the last 1000 miles have been on the Felt, and i have never really pushed myself on the Masi.
So this week the Felt went in for service, and i went out to do my usual morning 25 mile exercise.

what a difference!
Stiffness - i could feel the steel flexing by the BB; not so with the Felt.
Smoothness - the steel bike has a much "quieter" feel than the Felt.
Shifting - the steel bike has 105, and Felt has Rival. i've posted this in other threads - the 105 feels smoother, and the click between the gears is more subtle. good for this type of bike, but i like the clicky precision of Rival better when i'm pushing myself.

Saddle - Brooks on the Masi, Selle Italia on the Felt. The Brooks was an interesting change; sometimes it was nice to have less friction, sometimes annoying.

Pedals - my speed, especially uphills and at higher cadence, was noticeably slower without clipless pedals. i also felt much less stable, since my foot was moving around (even with the straps tied down.) I guess it's been so long since i really pushed myself on platforms that i hadn't realized how much of a difference it would be. also i couldn't stand that i was getting passed by people that i would otherwise be able to keep up with.

Bottom line:
(1) i bought another set of Keo's right away.
(2) the bikes are very different, which IMO kind of justifies the multiple bike ownership. each is a great tool for a different task or style of riding. i think that's awesome.