Tandem Cycling - CF seatpost shim: Thermoplastic or Aluminum?

I'm getting a CF seatpost 27.2mm and putting into my 29.8mm seatpost tubed frame. I'm using a shim for the frame and the stoker stem. Does it matter, or any advise on which to use? Would the thermoplastic crush, or slip or protect more than an aluminum?
Thanks
SB

Does it matter...

It shouldn't matter. In fact, Calfee frames all have a fiberglass insert at the top of their seat posts. It's there to prevent galvanic corrosion between aluminum seat posts and the carbon tubing, but it's there and it holds seat posts just fine.

In regards to how secure it will hold, I have no experience with the plastic type, but it shouldn't be a problem. Plastic however, can distort under a constant pressure, where as metalics won't.

In regards to the corrosion, the plastic is the better insulator between carbon fibre.

If you don't scratch through the seatposts outer resin layer, it will act as a barrier to minimize corrosion.

What is your total planned setup, frame, post etc. our Co-motion seatpost shim arrived yesterday. The size was not one that was easy to find being 29.8 to 28.6. Found that from a tip in a post here.

Now that it's here I'm reconsidering the options, going with two 27.2 Syncros Ti posts, which I have in two other bikes, instead of two Thomsons, one 29.8 and one 28.6.

PK

I have a 05 Raleigh Coupe, aluminum, which I got on super close out, new old sale. It's not the greatest, like some of the other bikes out there, but it happens to be just perfect for us, especially for my daughter in back. I got it b/c I didn't want to 'invest' until I knew we would use it. Good news is that we love it and I've done a couple of things to it to upgrade it a little.
Anyway, its got a 29.8 seattube, and I'm fairly long legged, so I need a long seatpost. I found an aluminum 29.8 with a 15mm? setback, 400mm long at a shop in England and got it. I like to have a lot of seatpost in the frame, and past/below the frame joint. I'm a big guy at 220 lbs, so its gives me more confidence with the frame/seatpost interface stength. Since that frame isn't made anymore, its cheap insurance. I have a ST thudbuster on the stokers position.
Now I'm looking for a CF seatpost, anything longer than 350mm, most likely a 400mm length 27.2mm and I'll shim it. I'll get 2 shims, which are easy to find in that conversion, one for the frame and definitely a thermoplastic for the stoker stem. I DO notice the difference in CF seatposts as far as comfort is involved on my single, so I'm going to try it for the tandem. I had tried Thomson, since I have them on my mountain bikes, but I really could tell the difference, meaning they were much harsher than the CF. (at least that's what my butt told me!).
The Ti idea that you have seems sweet. Do you feel you can tell the difference from the aluminum Thomsons?
Thanks for the input.
Now if I could find a used Bontrager CF 46mm drop bar................