Help to choose an Electronic Flash

[zenon5940]

I'm looking forward to buying a flash unit for my Nikon D90 and seeking advice in choosing the right one.
Nikon's SB 900 at close to $600 would probably be the best unit if $$$ were of no concern but I'm looking at the Sigma EF 530 DG Super presently available at $199.

Considering I could buy 3 Sigma EF 530 for the price of 1 Nikon SB 900 and that the Sigma unit offers more power ( guide #53 vs guide #42 for the Nikon ), is the Nikon really that much of a better unit?

When comparing specifications, the only meanigful advantage of the Nikon for me is that it offers off camera "intelligent TTL"...

Any advice would be appreciated.