oohh tricky one
I found the cotton ones went stiffer if line dried, but stayed nice and soft if tumbled (a little bold 2 in one never hurts either
), the bamboo stayed nice and soft whether line dried or tumbled. The cotton ones I had were a smaller fit than the bamboo ones, and the bamboo were slimmer and less bulky.
In terms of how well they last, I have to say I have had both styles from new and for as long as I had them the wear on them was minimal for both fabrics. I have never bought pre-loved in either so have no idea what prolonged use would be like on them