Circular needles are preferred by most long time knitters, and now that you have the hang of knitting back and forth on straight needles, you're surely ready to move on to better needles (IMHO).
Buy a cheap set up at your LYS and get used to them. I've got tons of circulars, with different sizes of needles and cables between them for different projects. I am now just going to buy myself a set of Knit Pick Options with the interchangeable needles and cables. It will clear our one drawer of my yarn stash... room for more yarn!