haha, good budget you got there. If u got the wallet, i'd say go for 580EX II. I'm also in the same dilemma except quickly cut short by lack of funds. easy: no need to think - no money do all the research also cannot buy. haha!
I started out with landscape in mind and i do not regret getting a good tripod. You should look at one if you do - you definitely need 1 for low-light landscape situations imo...or basically any thing slower than recommended shutter speeds XD. ie.. long exposure + slow-sync flash if shooting ppl at night. i dunno. endless buy buy buy