Depending on the pattern sts of the afghan, is how you would determine which cast on. But if they don't say anything except "cast on X number of sts", then I, personally, would just use the long tail. As for chenille-- it can be a difficult yarn to work with, and I wouldn't recommend it for an afghan. It tends to leave big loops in the fabric you've created and can shift and look as if you knit it on the bias. But the other issue is that it can be very "sheddy"-- do you want that in your afghan? Also, again, it depends on the stitch patterns used. If it's cabled, I would never use something like chenille, which will hide your stitch definition.