Heya,
Focal length and aperture being the same, the resulting image should look pretty close if all things are the same. The MKII is pretty sharp for a zoom. It's nearly as sharp as the really sharp primes. It's sharper than some other primes. But it simply cannot replicate the look of F1.4 or F2 for the same focal lengths. If the MKII were a longer focal length, it would produce similar results, kind of like the 70-200 MKII, which can definitely deliver some very good isolation and blur at 200mm F2.8. But the bottom line is using something 50~70mm on that 24-70, at F2.8, just doesn't isolate the way something with a wider aperture will. I know that's common knowledge, but it's the bottom line.
The 24-70 MKII is a great lens, it's all about convenience but with the added property of having really good image quality, and naturally, it costs a ton to suite.
I'm not particularly drawn to any of the 24-70's. I just have no need to have 24mm and 70mm on the same lens. And I certainly don't care for F2.8 on something that costs $2.5k. For that cost, I'd rather cover those focal lengths that I use most often, with primes that have F1.4 respectively. I'm not a convenience guy though.
Very best,