I've shot the Saiga 16" in both those calibers and the recoil difference is quite small. If you even think you want the x39, get it. If you ever want to convert your Saiga to AK configuration, a converted x39 will use any standard AK mag - cheap, easy to find, nearly indestructible. A converted .223 Saiga will require specialized mags that are not as cheap or easy to find and in many cases not nearly as durable (exception: Galil and Weiger mags - both scarce and pricey right now).
I know this is a bit cliche, but I find a lot of rifle platforms really work best in their original caliber; AR15 in 5.56, AK in x39, M14 in .308, etc.