I would say Venus is lightly better than Rena if not they are equal on this surface and on Hard, but Rena is slightly better on clay, but Venus could win some from her because she isn't a slouch on clay either. On grass topspin dosn't help as much as on clay and harcourt, thats why claycourters have problems and Kim does to.. The grass dilutes the bounce, therefore Serena's second serve is brought down into peoples strike zone, whereas Venus's slice gets lower and skids more, actually making her second serve more effective, plus Venus hits slightly flatter, comes in more and I think knows how to use a grass court better, where Rena knows how to use a clay court better, and then it equals out on hardcourts.
I will admit Rena is the most talented female tennis player ever (laj) but I don't get where she is that much better at hard courts, clay yes, the angles, and how Venus is better on grass, the net (that is a hude difference by the way, not a small one). The time the sisters have been on tour the difference was the mentality of them. Serena was more negative until 2002, and then Venus has been more negative since, but I think if both got over it then it would provide amazing matches on any surface, because they are the two most powerful playes of tour, and can hit and retrieve winners for either of them. Just my honest opinion!!