You will get varied responses I'm sure. But heres my take on things.
Flash is fun - but get it off camera. On-Axis is 95% of the time ugly. Off-Camera gives massive amounts of additional control and compositional elements to your image. Its also far simpler than it seems. (Doing it 'well' is another story..... lol).
I spent a fair amount of time sussing out how/what to invest in Speedlite wise... and heres what I came to.
TTL is nice - but expensive, and unpredictable. It'll add from $200 -> $500 to the flash, and you can fire off the same shot 5 times and get 5 different exposures. This equates to very low 'repeatability' or 'control'.
The advantage of course is, you can *potentially* - forget about controlling the flash and just 'run and gun'... hoping TTL gets it more right more often than not.
Also - the cheaper TTL controllable flashes endup providing the expensive TTL bit by usually greatly reducing power, which means the Flash-to-subject Distances you can work with are greatly reduced. I can easily light someone 10M's away with a '560.... you'd be hard up doing that with a AF360 or AF280 (which puts out about half the light that a YN-560 does.....)
The other side of the coin is - Know your Flash! A few evenings playing at home, and you'll have it fairly sussed in terms of controlling the flash, and thus controlling the lighting elements in images. Using a manual flash not only lets you 'appreciate' more what you are doing and why - but its guaranteed repeatable. Light *always* behaves the same - given the same parameters.
I have a couple of YN-560's and some Sigma EF-500DG's and a 530DG... The 560's are more powerful, and 1/5th the price. They also have PC-Sync ports etc. And a much better Optical Slave. YN-560's can be had for about $70... And the beauty of Manual - It'll let you pick up any flash - for ANY camera - and be confident in operating it. TTL is a crap shoot, and also very different between Pentax, Nikon and Canon... so if you are tied to using TTL, you can be just as tied to a platform as much as a particular Lens Mount...
Anyway - Just my irrelevant thoughts...
Good Luck.. get some radio triggers tho, and get Off-Camera... it'll change the way you think about pictures (well it did for me anyway
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