Safari 5.1 is available for both Snow Leopard and Lion, but there are notable improvements in Safari for Lion that makes it a better browsing experience. These Lion only improvements are made possible due to the new features in the underlying OS X Lion, and are good enough reasons for some to upgrade to Lion from Snow Leopard.
Using Safari in full-screen mode is just a pleasure. Each window and its tabs occupies one full screen, and you quickly switch between multiple full-screen by a quick three-fingers swipe. You can have one or more Safari windows in full-screen mode with the rest remain as original size within one desktop.
Multi-touch gestures gets an overwhelming emphasis in Lion with new gestures for switching spaces, access the mission control and launchpad etc. For Safari, new in Lion is the ability to double-tap to zoom and magnify part of a page. This is similar to what you can do on iPhone and iPad. You can also use the two-fingers swipe left-right to navigate forward and back the visited pages. And you switch between full-screen mode and other full-screen spaces by swiping three-fingers left or right.
When you mouse over a word, you can bring out the dictionary and thesaurus of the word by simple tapping on the word with three fingers. This is more convenient than the keyword shortcut Control-Command-d.
The Download window is a popover window for Safari on Lion. You access the download popover by clicking on the download button on the right of the search bar. This is how download window should be.
Besides all these changes from the Snow Leopard version, we notice Safari on Lion uses less memory. We have not conduct any actual test, but based on our normal browsing pattern, Safari for Snow Leopard consistently hits over 800MB to over 1GB of memory usage. In Lion, the memory is hovering in a healthier 500MB-700MB range. This is not a conclusive result, as memory usage depends also on how many and what type of plugins you are running within Safari.
We have to conclude that Safari is the best browser right now for Lion with its full-screen mode, multi-touch gestures support and features such as Reading List and Reader.