I'm split, for me it depends on the location.
If in a city/woods (Slightly crowded area), Hei's assassin skills will win it out easily.
If its on a plain or a clearing, I'm saying a tie - leaning more towards Shizuo.
I think Hei's skills will allow him to dodge Shizuo's punches and whatnot, but at the same time I can't imagine him getting close enough Shizuo enough times to finish him off effectively. So thats why I think it will just end up with both of them being worn out. However, being in the clearing/open area, Shizuo has a slight advantage since he can easily track Hei.