I’d say if you are a casual iPad user, then your not going to notice anything drastic from a 2nd gen to a 3rd gen iPad Pro, other then a new design, and the obvious differences, and visual design changes. If you have the cash to upgrade, then why not I guess?
But, I’d say the largest difference is with the 1TB models. You get 6GB of ram, and this really helps with productivity applications mostly. BUT!
If your not using your iPad for any serious work, then that wouldn’t matter much to you anyways.
The A10X vs A12X in light single threaded applications are going to both be very fast. Although, it’s when doing a lot of heavy rendering on the A12X, then it can pull drastically ahead. And if your not doing this, then you probably will not notice much of a difference.
I guess it’s kinda like getting a new truck that can tow twice as much as your old one. But, you never hauled anything heavy in the first place.
I can see someone using a A10X for 2 years rendering through footage and encoding files on a daily basis. And then they upgrade to a A12X. They will probably instantly notice a difference in there workflow. But surfing the web, watching videos, and gaming. It’s not like you always need the best or bleeding edge performance for this. As the A10X will handle just about anything just fine!
Other then wanting the newest, or the best iPad available. Which is usually the only reason most people will upgrade to the iPad Pro 3rd gen.
Other than that, apple products last a long time! And that’s why I bought the iPad I have. I plan on keeping it a while, it has 1TB of storage space, and 6GB of ram. But, I’ve never owned an iPad before this year. And if I had a iPad Pro 2nd gen, I’m sure I’d be happy too!
My honest opinion outside of newness, and just getting something new because it is cool is. Well, the iPad Pro 2nd gen will continue to suit you just fine!