Even the name sounds better...nissin sounds like the instant mee i ate for lunch. 580ex2 vs metz 58 more fair comparism. Its like comparing a nissan with a bmw. The 580ex2 is more like a lexus so its more comparable
I hv the Metz 58 AF1 for basic normal use as I'm a hobbyist. The reason I got a Metz was b'cos my photography coach strongly recommended it and he's a wedding photographer so he shd know what he's talking abt. As for the Nissin Di866 it's also a good flash supposely to compete with Nikon SB900 or Canon Speedlite 580EX2. But I suppose the Nissin is value for money lor.
Do read more pros & cons on both Metz & Nissin Ultimately it is what purpose you want a flash for so decide wisely