Interesting question. Without absolute lab testing and a lot of engineering mumbo-jumbo, we'll probably never have an absolute answer...and even then you'd have to test every fork and bike headtube in existence. But we probably have enough real world experience to make a logical assessment.
I don't think the fork's steerer size...1.125 vs. 1.5...makes a big difference until you get to 150mm or maybe even 160mm. Design and materials have yielded some very strong and reliable 1.125" steerer forks. At 140mm I wouldn't be concerned with the rigidity of a 1.125" steerer. Of course all this is based on a bike with a solid headtube and a quality fork.
On the axle size, however, I think most of us who've ridden all manner of axle interfaces can agree that solid axles, even the 15mm, is generally always going to deliver better steering precision. Again however, this is somewhat dependent on the wheel stiffness and fork stiffness. For example, a 30mm stanchioned Suntour fork with a 20mm axle...a stretch, I know...would probably still be a wet noodle.
In your case I'd go with the 140mm Talas with 15mm axle as being noticeably stiffer. I don't think the F series Fox has as stiff a chassis as the Talas you're referring to. While they're both 32mm stanchioned, the arch and other design elements on the F are not as substantial. Remember though, that how and where you ride will also be a big part as to whether you'll notice a big difference.
