Jarv - I have full sets of both, and use very frequently. I would venture a guess that the soft .6 (2-stop) is by far the most often used, followed by the soft .3 (1-stop). Without knowing what type of scenes you mainly shoot, it would be hard to tell you whether the soft or the hard would be your best choice for starting with. If you most frequently shoot flat horizons, personally I'd start with two hard grads... if most of your scenes are of mixed variety (ie: hilly or mountainous), I'd suggest starting off with flats. Generally speaking, the 2-stop filters will serve you "most of the time."
A note on quality: there's quite a large difference in some filter brands. The ever-popular Cokin filters are NOT "neutral" in color, and they're honest about it... I believe they call them "Gradiant Gray" filters, and they do affect colors. I would suggest staying with HiTech or Lee or Singh-Ray filters of this nature... they are indeed "neutral" and are better optically than Cokins (which do make some fine special effects filters). Of course, these brands are also considerably more expensive than Cokins, but in my opinion, well-worth the added expense.