I've run an eight speed M7040 hydraulic shuttle some, mowing pasture and moving round bales mostly.You've owned and operated both and all you can say is what is completely obvious. Was the 8-speed a problem mowing the same parcel compared to the 12 speed? Did you often have to stop and change range with the 8-speed but didn't with the 12-speed? Mowing is a low bar in terms of these machines so you couldn't stay in high range with the 8-speed and four forward gears weren't enough?
I think creep option is absolutely relevant but you're entitled to your opinion as I am to mine.
As for 8 vs 12, I don't think it's going to change your range shifting. Aren't the six speeds spread over the same MPH as the four? More speeds would allow you to maintain engine/PTO speed, but I think the cutter still cuts fine at 350 or so, and this size tractor has plenty of torque for a 7' cutter. If you were baling hay, that may be different. Eight sure beats the 3 I started on!
With that said, if I were buying new I'd check out the 12 speed. But if a good used 8 speed showed up, I'd give it a good look too. Kinda depends what you'll be using it for.
Are all the creep gears slower than low range? If so, tilling is about all I could see it being used for.