Thanks Kevin for all the practical advice, obviously learned through lots of experience. I noted your comments regarding shutter speed, what is the lowest speed you have used that will still yield a decent percentage of sharp images?
I am thinking about using my 50mm f/1.4 lens, I will do some ground tests @ f/2.8 first to see how it performs.
50mm f1.4 not very good unless stopped down a reasonable amount. Apart from being soft it makes lights smear, looks like you shot through glass with raindrops on. The 24mm f1.4 mmii wide open is awful in the corners, even the client noticed.
Favourite is the 35mm f1.4 stopped down 2 stops.
You will be surprised how long you can hold onto 1/100th of a second as the light fades, I've been down to 1/30th the percentage of failures is greater than success at 1/30th, obviously conditions play a big part. A lens with stabilisation helps too.
Everything is telling you to keep upping the iso. I would go find a hill over looking a City one night and try it all out, that's what I did. I did it with my 1DsII, 5DII and a Nikon, famed for it's low light capability.Use different lenses, stops, iso etc You can check how the AF works as well. I had no luck trying to focus on a pinpoint of light, that fooled the af. I know standing on a hill is not the same, but you get to find the limit of your gear so you know when increasing iso will be better than blurred images.
I was talking to one guy, he had a pair of ks6 mounted at ninety degrees to each other, he reckoned he had great success at 1/15th.
I would hate the money shot to depend on getting it at 1/15th. If you can hire the KS6x6 I think it would do a really good job for you.
The rig I have just built probably would be good at 1/15th but I would rather be 1/60th. It's photography all round-a-bouts and swings, you gain in one place by losing in the other. Go shoot handheld from a hill one night, if you can do it with a gyro all the better.