Hi Joe, the four (might be five) 206s I've got are all aluminum. I've heard that they made the 206s in cast iron as well, but I've never come across one. The one 306 I've got is also aluminum, and I thought that they all were, but I'd read online that some people own heavy, cast-iron 306s.
I'm not sure why it is that I only seem to come across aluminum 206s and 306s--I wonder if the cast-iron ones were not made in the 'States, or if it's just particular to my region (NYC), or if it's just dumb luck. If anyone can shed any light, please share!
That being said, I'd never take a cast-iron 206 or 306 over an aluminum one; having to tilt the entire machine back to change the bobbin is already a hassle, particularly if you're mid-stitch and still have the work under the needle--it's got to be an even bigger pain when the machine is heavy cast iron.