I want a 24-70 f2.8 quite badly (I'm leaning towards the Sigma, since it's half the price of a Canon L)... But I'm worried that at wide-open it's gonna be ever-so-slightly soft, and stopping down to f3.2 (or whatever is next) would be just a little too dark for night stuff.
As such, I'm having a hard time deciding whether I should just get a set of old nikkor primes (I'd need a 24, 35, and 85 at ~f2, which I could then stop down to f2.8 for additional sharpness, but primes are usually sharper than zooms anyway), or just bite the bullet and get a 24-70. On APS-C, I'd almost think that 85mm (equiv. to ~135mm) might be a little too tele, so I could probably skip that. Conversely, I might need/want something wider than 24, but similarly fast (I don't think Nikkor has an old 20 f2, do they?)
I'm also looking at the Sigma 20mm f1.8 and 30 f1.4 with lusting eyes, but I heard they're APS-C only, so if I ever go to Full Frame I'll get poopy vignetting (a big pet-peeve of mine... I don't like it as a stylistic choice at all.)
...By the way, I'm coming at it from a video perspective (which is 1/30th shutter and ISO1600 maximum.) On an APS-C, 70 is equivalent to approximately 115mm, which is a pretty decent tele.