The Vivitars have great reputations, but you might also look at the Metz line. While the large "potato masher" 45s (I have the 45 ct2 with TTL capability) may be cumbersome with the pistol-grip already on your cameras, Metz has smaller units that might work better with your setup. Some also support standardized TTL modules that can be changed according to the camera it's being used with. Having a unit with both tilt and swivel on the head is handy.

I'd agree with Martin, though - more power is better, and more flexible - as long as the unit allows you to dial down the power to fit your environment. As far as I know, all of the Metz units have that feature.