When my new frame's built up (next week sometime) I'll be able to do a direct carbon/alu comparison, since a friend has both a carbon and alu post in the right size and I'll be trying them both out. I really want a Thomson setback post for it, for looks and adjustability and safety and to match my stem, but if the ride with an alu post is too harsh I'll get a carbon one.

The frame is ti, so I'm confident it will be OK with the alu one, but I do seem to have a pretty sensitive rear end ...