Re: Peak Henin vs peak Venus
Justine obviously wins the clay match and Venus obviously wins the grass match. The hard match is difficult to assess. Justine's fast court skills are often underrated, but there are also a whole slew of speeds for different hardcourt surfaces.
Looking at Venus' peak 2000-2002, she compiled a 90-5 (.947) record on hardcourts, winning a whole slew of important titles, including back-to-back US Opens, the Olympics, Miami, and going undefeated in the American summer circuit twice.
From 2006-2008, Justine went 57-3 (.950) on hardcourts. She played a lot less, but also had some amazing wins, including two Year-End Championships, Toronto, and the US Open. Justine's record becomes a little shaky because she played very well on hardcourts in other periods of her career, but not necessarily when she was at her peak, such as the Athens Olympics and her second best year in 2006.
I don't believe two players can play at their absolute best against each other at the same time, as the sport of tennis works in ways that if you are playing your best, the other person is stopped from doing so, as well. But the 2007 US Open SF match between these two isn't the worst indicator; Venus wasn't at her peak in this match, but she was definitely not riddled with the disease (she ran and ran for days and didn't appear winded) and she wasn't injured.
All in all, Venus wins in tight matches on both hard and carpet.