Flash for macro (bugs and flowers)
Since you have no other flash yet, it makes sense to get a 580 (or other "regular" flash) as your first one.
It'll come in handy for a lot of situations. A special macro flash is a great thing to have, but it's not as flexible, so to begin with, unless you're really into macro, the 580 seems like the right choice.
I've gotten a lot of good use from my 580 for macro shooting.
There are a lot of easy to build things that you can construct to diffuse and direct the light from a standard flash to where you want it to go for macro shooting.
Another accessory that you may wish to think about, along with the 580, is an off-camera-shoe-cord (OCSC). That will let you get the flash out of the hot-shoe, and position it differently to achieve different flash illumination for your macro shooting.
Often, an OCSC is used with a flash bracket to hold the flash in the desired position. I've shot a lot of macros, actually, by holding the flash in one hand and the camera in the other. It's something of a pain, but it works, and is the fastest way to adjust the position of the flash and the camera to get just what you want. But there are special brackets for macro shooting that would make life a lot easier.
A flash diffuser is also a great accessory both for macro shooting and for other uses. So you might want to build one or two just to play with that as well.
For general shooting, having a flash bracket and an OCSC allows you to position the flash more quickly when switching between portrait and landscape orientation while still getting the bounce angle the way you want. But you may find that you want one kind of bracket for macro shooting and a different one for your other shooting.
I highly recommend the MT-24 EX flash for macro work, but I would not recommend it as the first flash you buy because it is really meant for very close-in shooting, and won't do much for you for all of the other uses you'd have for a flash. On the other hand, something like the 580 is great for macro AND for other uses, so I think you're on the right track getting that kind of flash first.
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