Based on my not-so-vast experience:
Doubling up can be a mistake - I attempted to do that hot damn afghan thing, and it ended up being way chunky and ugly. You completely could not see any stitch definition. Perhaps this is the reason why you never see actual pictures of a completed hot damn afghan out there. Yeppers.
I have several crocheted afghans and I can guarantee that they were made out of the cheapest yarn available at the time. (as the lady who made them had extremely limited resources). They wash up and do lose their plasticity and they wear like iron. They are fabulous.
For your first attempt at an afghan, I wouldn't invest too much - what if you burn out or get tired? You don't want that bag of $100 worth or more of yarn staring you in the face giving you the guilts.
I am voting for simply soft for you. I have skeins to do ripple quilt with homespun, and haven't started because I think it will be a PITA to work with. (also because I am trying to finish the slowest ever sweater) Whereas the simply soft is supposed to be delightful. Once #1 son picks a college, I plan on making him an afghan in school colors (I also plan on making him a quilt so the little pissant better pick a school quick). Here is a place with inexpensive simply soft: