The most important fact is that both OS is on the right track because everybody is gaining something on both ends. Snow Leopard and Windows 7 will bring a very close battle in a war between Apple and MS and that only benefits the customers because for the first time in recent history, both OS may turn out to be very good and desirable for both sides. It only encourages innovations from both ends for the next OS. W7 has something SL dont? Apple will come up with something better for 10.7, SL has something W7 don't? MS will copy it.
MS lost the battle between Leopard and Vista in the never-ending OS war (until one company goes out of business of course) because Vista did suck in the beginning with horrible drivers support and so on. This resulted into a bigger market share and better company image for Apple. While they are still gaining more people, the question now is can they continue the momentum into the new battle between SL and W7?
Windows 7 is coming back strong and so is Snow leopard. This battle is going to be VERY CLOSE and I honestly think Apple won't win this round easily as it did with Leopard. The economy may push the win over to MS this time, with the W7 running better than XP on netbooks, people are going to want it even though netbooks are still a small market. Combine that with HPTC, slim PCs and low ends PCs that could run W7 better than Vista and XP, definitely enough of an impact on sales that W7 could sell better than Vista ever did at faster pace.
Snow Leopard is going to be an awesome upgrade too. Small changes all over; Exchange built in support, OpenCL/Grand Central, pure 64 bit optimized and leaner OS, data detectors, system-wide text manipulation support, and so on.