Opinions on the RF 28-70/2 vs RF 24-70/2.8 in terms of IQ?

PGSanta wrote:

highdesertmesa wrote:

With IBIS coming and their IQ being so close – cost, weight, size, and filter expense are the only remaining and substantial differentiators.

Like you said, f/2 is THE reason to choose the 28-70, but it is difficult to underestimate the difference that makes in the image character. The 28-70 @ 70mm/f2 renders like a Zeiss prime, which is quite astonishing for a zoom.

My 28-70 was a tough return, but I did so because I just didn't think it offered a close enough rendering to a good 1.8 prime (let alone a 1.4, or 1.2). I sent it back on the hunch that id' be happier with the IS zooms when I needed a zoom, and primes when I needed speed. I'm happy I went that route, as the 15-35, and 24-70 that Canon has released are fantastic.

I think the only real way you'll know for sure if the f/2 is something that will impress you, is to use the lens. Some folks love it, with good reason. I feel it's an impressive design, but for just about 99% of scenarios where I'd be shooting close to f/2 I'd rather use a fast prime that would almost certainly give me a superior image.

I get and agree in part with what you are saying. But to me the only real ready to choose the 24-70 over the 28-70 is... the 4mm on the wise and, and weight/size, which can be offset by the f2.  
Once IBIS is out, the IS in the 24-70 becomes less of an advantage (I mean 7 stop combined IS vs 5 stop IBIS can be a bit of a differentiation). After all the cost of both lenses are relatively close. 
Regardless though, the f2 speed can simply mean lower ISO settings in dim conditions where IS doesn't help which translates directly to better IQ (if you can deal with a shallower DOF) and potentially also improved AF.