The dpns will definitely be easier to handle in bamboo, for a circ, it's up to you. I don't think you need to consider an interchangeable set just yet - you can get a couple more needles as you need them. And with more experience you may find you prefer knitting things which take smaller sizes, or may prefer larger sizes. You can use circulars to knit flat with too, something around 30" in length is good for most things, adult sweaters and small blankets as well as flat scarves. And you can use that length for the magic and single looping of smaller things like hats and mittens. After you see how your first one goes.
sue- knitting heretic