You have no idea what you're talking about!
The 4GB RAM on the 12.9 is so that Pro applications such as photo and video editing apps can run without any memory related crashes etc.
It in no way will speed up or slow down a device during general usage.
The iPad Pro 9.7 will become unsupported at roughly the same time the Air 2 will. 2GB RAM on the 9.7' is just so that Apple can discontinue it earlier.
The 9.7' has 2 cores while the Air 2 has 3. There is a very marginal performance increase, and pro apps won't perform any better because Apple skimped on the RAM. It would simply be foolish to upgrade to the Pro 9.7' unless you REALLY need the Apple Pencil.
Haha whatever you 12.9 guys need to justify it to yourselves. No app crashes on my Air 2. Anyone who upgrades to the 9.7 will be very happy with their purchase.