2010:EE is 'the next step', it includes everything 2009:WBE did, and then some more features - speedtrees, layers, the ECML route.
The only real reasons to buy 2009:WBE instead of 2009:EE are:
1) Cost, WBE is cheaper, slightly - but more importantly, there are more vendors/sources of it than 2010:EE which requires buying direct from Auran
or
2) 'Speedtrees are bad, mkay!' (This requires ignoring the fact that you could just not use speedtrees if you don't like them, and they ARE a visual improvement over the older trees, especially with SP2 where they don't have to blow about so much)
