Started a similar thread over at DPReviews.com. Posting this here as well, to get as much good advice/many good answers as possible.
Just watched a few YouTube videos on flash photography, and found them to be very interesting. I've never used external flash before. I have a Canon EOS 60D and a Fujifilm FinePix X100, and I mainly shoot portraits of friends and family, as well as my dog (using a 50mm f/1.2L and a 24-70 f/2.8L), in addition to some street and landscape with my X100. I'm looking into buying my first external flash (would love a multi-flash setup later), and would like some advice on which one to get. This is mainly for my 60D and portrait/dog photography (compatibility with my X100 would be nice, but not at all required). I don't mind paying more for more features, but if a flash has features I'm never going to use, I'd rather spend my money elsewhere (or on several flash units).
Apart from specific advice on what flash to get, I have a few other questions:
i) What do I need for them to sync wirelessly with my 60D? (Does this require a 'master' unit sitting on the camera?)
ii) Are Canon Speedlites better than third-party alternatives in any way?
iii) What should I look for, specs-wise?
iv) I can afford a 600EX-RT, but I'm guessing there are features I'm guaranteed never to use, and that I'm better off spending money on several other units?
v) Are there any 'must-have' flash accessories?
Thanks a lot, and I apologize in advance for all the questions!
My two cents.
Only real advantage Canon Speedlight has over third party is ETTL - which tells the flash the "correct" amount of flash FILL power to throw unto the scene.
Seeing as you will be photographing moving subjects then this might be a good advantage for you. I'm not too familiar with the 60D but looking at a quick glimpse I see it has no pop-up flash which means you need a master for Off-Camera. With that fact alone, I would go with the Canon 600-Ex-RT. ATM it's not much more than the 580 plus. You will also want the ST-E3-RT at minimum. It will trigger the 600 pretty reliably from what I hear. This will all come out to about $900. Instead of the ST-E3-RT you can do a second 600-Ex-RT to take advantage of IR focus assist (patch coming soon) and extra fill.
To Add: You don't NEED extra modifiers but I would recommend a Rogue Large FlashBender + Diffuser Panel ($45 for handheld Off-Camera, plus it's sweet how versatile it is); a light stand; diffuser umbrella (has a removable cover); umbrella adapter; and a 5 in 1 reflector.
Also re-reading your post, the 600-EX-RT is definitely the way to go if you want to add more flashes later. We should hopefully see a 430 - RT version of the less expensive flash.