the efi will get better gas mileage. personally i wouldn't even consider a z950. if i was gonna spend that much i would get a scag cheetah instead with 61" deck. it comes with suspension platform and 16mph transport speed and probably at a cheaper cost.
The 950 was their 31 HP option. Now Kawi has it shown as 27 HP under the new system of measuring HP. It is not one of the big block series of engines that were known for high fuel use compared to the rest of their line, and I've never hard anyone claim that the 950 was really bad on fuel useage. I'd take one of them long before I'd pay all the extra money for the EFI (which is now rated at 24.5 HP). It'll take quite a bit of fuel to make up the difference in cost of the EFI. Just MO
We have 1200 hours on our Z950A. Both of these mowers are great choices in my opinion. I would not get the scag cheetah, I demoed one and the hydros were stiff, it was hard to see the wheels to make sure you aren't spinning and it felt huge. I love our Z950A, but it does really suck the fuel. I would see if I could demo the Z925A EFI on a wet day to make sure it has the power you want and if it does or it is close I would recommend that.
With todays fuel prices go efi on a 60 inch mower it will be fine. A deere deck requires less hp than exmark or other mowers. It flows air much more freely thus only cutting the blades a few times compared to exmark which re cuts clipping more finely which robs power. I have ran almost every mower and this come from experience. I have a 970 and it uses some fuel but also cuts a lot of acres!