That depends. You get Lion for free anyway now, if buy the Mac now. If you buy now, you get a Mac with a stable OS and can upgrade whenever you want. If you wait for Lion and purchase then, you get a not so stable OS and can't switch to the now stable 10.6.8.
And as the Mac mini, MBA, Mac Pro and MacBook will only be updated after Lion has been released, you only get the older models now, and if you want a newer one, you only get Lion.
But Lion is alright for most users I guess, thus it depends on you.