550 EX question.

I have a chance to pick up a brand new 550EX for the same price as a 430EX.

Is there anything it lacks over a 580EX??

Not anything that would make a significant difference (other than cost). The 550EX has an insignificantly less power output than the 580EX. It recycles in 4 to 6 seconds vs less time for the 580EX. But the 550Ex costs less, too, which may not be insignificant for a lot of people.

Using the buttons on the back of the 550EX can be a challenge for people with large fingers.

One of the favorite 'pros' of the 580EX for most seems to be the quick dial control. The second most frequently discussed seems to be the size. The 580EX is smaller than the 550EX.

The third 'pro' I seen discussed is battery usage. Not only does the 580EX cycle faster, it also seems to get more cycles out of the batteries.

Now let me quantify this; I don't own a 580EX. I have two 550EX's and don't anticipate buying anything newer for quite a while. I find the 550EX gets the job done quite nicely.., and it should. After all; It was Canon's top of the line flash before putting out the 580EX.