My first circs were 16" in sizes 5, 7, and 8. It was winter hat season, so that worked well. Then I learned magic loop and got a bunch of 40" sizes 2, 3, 4, and 5.
In general, I think I'm a fan of the longer cable, only because with that one I can do the most with magic loop. With the 16" circs, I found that for the smaller diameter work (like the decreases at the top of a hat) I either needed a second circular needle, or switch to DPNS (which I loathe). Seemed smarter to get 1 40" inch needles, money-wise, since I'm not doubling all my purchases.