I have a Bu-Jin cotton/poly blend hakama made of the heavy (8 oz) cloth. It is quite comfortable and the fabric breathes well.
Based on what I've seen, the cotton/poly blend is much more durable and color-fast than 100% cotton. The blend doesn't shrink and holds pleats without heroic steam iron work. So, if you get the aiki-style koshita, you can wash your hakama like any other garment.
Best of luck, and take good care of whichever hakama you buy. It's as much a training tool as your weapons.