I started with Souten but there wasn't any reason for it. Just randomly, though I knew Hokuto was finished. One way to look at it is that you get the manga-ka after he's done growing but you don't know how influential this prequel will be(probably not any).
If its just to find out if you like the style, it doesn't matter which one but the quality of Souten might be better earlier on. I can tell you that Souten starts out slower so far. More story driven and the main character isn't as immediately brutal.