In my opinion, you can't go wrong with either (I like both). I've been using Lion extensively for almost a month now. It's been a super smooth transition for me. Things about Lion which I have found to be a vast improvement over SL:
-- The interface isn't as colorful as SL. In comparison, the Lion UI looks a little drab and washed out. But that's the point. Lion just recedes into the background to let you totally focus on what's inside the window. If I went back to SL, I think I'd find the UI very distracting.
-- Zooming in and out, a feature I use all the time (yes, I probably need glasses), works even better with Lion.
-- Super smooth two finger swiping to go back and forth from page to page within an app, while retaining three/four finger swipe to move from app to app.
-- "Show path bar" in Finder.
-- "New Folder with Selection" in Finder lets you group documents together and move them as a group.
-- Letting me assign a different desktop to each space really makes it seem like I have different spaces rather than different copies of the same space. Having different pictures gives each space it's own unique feel.
-- As someone who forgets to save, auto save and versions are godsends.
-- Those with iMacs as well as those who use peripheral monitors have been bitching about full screen apps, but for those of us who are on notebooks (mine has a 13" screen) and who don't use monitors, full screen is a major plus. I love the way full screen automatically opens a new space. It would've been a huge mistake to have new full screen windows opening in the same space. Mail is so much easier to read in full screen.
-- The new "change item arrangement" feature in the Finder window is great for sorting and ordering all your stuff by kind.
-- Power users have piled scorn on All My Files, claiming it's for people who don't understand how files work in os x. However, for the average user it is a quick way to access your files, especially when you forget where you placed a particular item.
-- The scroll bars only appear when you scroll. Some people miss them. I don't. The fact that they are gone makes windows look less cluttered. Again, this is probably welcomed more by macbook users than imac people.
The most annoying thing about Lion for me is a very minor but irritating thing: When secondary clicking is activated as tapping with two fingers, the secondary menu pane keeps popping up when you scroll with two fingers. The machine all too often reads two finger scrolling as two finger tapping. Although it can be disabled, the alternate method of reserving a corner of the trackpad entails its own annoyances -- namely, losing access to that area of the trackpad for standard clicking.
