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Am guessing that the other difference would be battery life. I can get approx 60 full power shots out of the Metz before the battery has had enough - thats a new battery as well. ..
There are ways around this. I have external SLA 6V batts (I don't remember how many AH) which I've wired into my Metz batteries. These last about 2-3 times longer than the standard 4.2AH batts. In fact in the last 4 years of using them this way I've never run out of juice and I drive them very hard. But yes, this makes them even heavier and more cumbersome, but the mod only cost about $40 per battery to do (not including the battery charger for the SLA batts).
The Metz is an ideal flash for certain kinds of photography because the next 'level of flash' (and I dont consider the Quadra to be that, being different not better) is far more expensive, physically cumbersome or both.
One significant disadvantage of the Metz, especially when trying to overpower daylight, is that the Metz has a very long flash pulse at full power, approx 1/125 sec. If you shoot at 1/160 then you will already see a slight reduction in power and this increases as you increase shutter speed towards 1/250th or even higher if your camera can cope. This is where other flashes can kill the Metz as their shorter durations are not truncated by the shutter speed being used. I try to shoot at a max shutter speed of 1/160, some times 1/200th, but I try to stay at 1/125 to get the most from the Metz.
The argument that the Quadra has a 'better' light may be true but ONLY for certain applications, ie with modifiers. Many people, like myself, use flashes without modifiers because they suit the subject matter. Shooting cars with brollies, softboxes etc simply doesn't work, for example. To me 'better' light is the focused light of the Metz 60, because it's a more efficient use of the available light.