I have a good opportunity for one of these.
What do these offer over my Z3 4-12x50 besides a broader zoom?

probably more reliable adjustments, different spring arrangement with 4 coil springs. The Z3 4-12x50 is one of the least reliable scopes I have ever owned.

My Z3 is on a 308.. It's held zero flawlessly.

I mostly was wondering about the glass... It almost seems like they use the same glass, so there may be nothing gained in clarity, brightness or low light.

I don't woods hunt with the gun so I don't expect to use the low magnification much at all.

I called Swarovski service...

According to the tech, the glass, the housing tube and the reticle are identical. The key difference between these two scopes is the erector set. Leaf spring vs coil spring. They opinioned on a 308 I'd probably never know the difference in performance.

Between the 2.4 -12 as compared to the Z5 3-18, is the only difference a gain in magnification at the low end and a loss of 6X at the low end?

They also have more adjustment then the Z3's and the light transmission numbers are slightly higher , slight edge on eye relief. The Z3 might be brighter then the Z5 based on what I have seen. I am interested in the Z5 model you have posted about as well. I like the Reticle choices better on the Z3 though. I have had 3 Z3's 1 AV in 4-12x50. Everyone was sent off , everyone of them crapped out with less then 20 rounds put thru it. 25-06,7mm-08,7mm Weatherby and 300wsm. Everyone of them came back fixed and they then went down the road. You wonder why I kept buying them? Because I thought I had the bad luck of the draw and that the next one would be perfect. My brother owned a 4-12x50 BT which he mounted on a Vanguard 300 WBy mag and never had one single issue. I had a 3-10x42 mounted on a 7 mag , never had an issue, just did not like the BRH reticle .

Go Figure