Would 1066 mhz at 6-7-7-18 be better than 800 mhz at 5-5-5-15?

On what motherboard/chipset do you like to use them?

It depends on what settings it'll actually run, the speed of your FSB (if there is one...) and other factors.

But all by itself, the latency is basically the same (one extra cycle but at higher frequency, or less cycles but at a lower clock, or about 12ns each). The 1066MHz has a little extra bandwidth though.

Not that you'll really see any real world difference between the two, unless you spend your days running memory benchmarks or something.

I'd go with the better quality RAM (better cooling, 1.8v or as close to it, better brand and warranty, etc) or better priced one instead of choosing based on the very minimal difference in performance between the two.