assuming the frame dimensions are the same, I cant imagine that your ride quality would differ much considering your on 4 inch wide tires, its going to be bouncy no matter what. that much cushioning will negate any frame material characteristics.
Id say base your purchase on price, color, and personal preference, anything but frame material.
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2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
2000 litespeed Unicoi Ti, XTR,XT, Campy crank, time atac, carbon forks