Apple spent part of its WorldWide Developers Conference this morning detailing Snow Leopard, the upcoming version of its Mac OS X operating system -- including plans to release it in September as a $29 upgrade for people running Mac OS X Leopard. That's about month before Microsoft plans to come out with its new Windows 7 operating system.

Microsoft isn't yet talking about pricing for Windows 7, but early clues suggest the cost of upgrading to it from Windows Vista is likely to be more than the price of upgrading from Leopard to Snow Leopard. A leaked Best Buy memo, obtained last week by Engadget, said the retailer would presell Windows 7 Home Premium upgrades for $49.99 and Windows 7 Professional upgrades for $99.99.

Mac and Windows fans will debate the relative value of the two operating-system updates, but the bottom line is that each is an incremental advance over its predecessor, said Michael Gartenberg, vice president of strategy and analysis at Interpret LLC in New York.

It remains to be seen how the difference will impact the market, he said, but Apple customers will probably end up happier than Windows customers when they see the upgrade prices. "Apple’s announcement is a much more realistic assessment of how an interim OS upgrade should be priced," Gartenberg said via phone after watching the WWDC keynote in San Francisco this morning.

At the same time, Microsoft and its partners are clearly adapting Windows 7 upgrade pricing to reflect the economy. The $49.99 Windows 7 Home Premium upgrade planned by Best Buy compares to a standard $129.99 upgrade price for Windows Vista Home Premium.

Bill Veghte, the Microsoft senior vice president in charge of the PC Windows business, didn't offer or confirm any specific numbers at a UBS technology investment conference this morning. However, in terms of pricing philosophy, he said the idea is to make the shift from Windows Vista to Windows 7 "a very smooth upgrade."