You are clearly comparing the new iPad Air and current iPad Pro models.
The iPad Air4 does have the most recent A14 processor - which, on the basis of published specifications and synthetic benchmark figures, may be slightly more powerful on paper. The A12Z of the iPad Pro remains an advanced powerhouse with more CPU cores. In reality, speaking personally, I wouldn’t use this alone as the basis of an objective comparison; the principal benefit of the A14 (7nm) CPU is that of improved power efficiency. If you intend to use the iPad for serious work, such as image processing or video editing, then additional RAM of the iPad Pro (with A12Z) is important.
The principal differences between the current iPad Pro models over the new fourth generation iPad Air are:
- greater screen screen size with 120Hz refresh and up to 20% greater brightness
- four audio speakers - L/R adaptive for screen orientation (iPad Air has only two speakers on the bottom edge)
- enhanced camera and sensor array - includes Lidar sensor
- FaceID (iPad Air4 is TouchID)
- 6GB RAM (the iPad Air4 has only 4GB)
Whether the major additional features/specifications (as bulleted above) justify the additional cost, is a decision that only you can make. If you are looking for the device with the greatest flexibility and features, then the iPad Pro is perhaps the better choice.
Obviously, the A14 CPU is the current generation 7nm technology that we are likely to see in the next refresh of iPad Pro, but until new models are announced, nobody outside of Apple can confirm this. As for longevity, at this time A12Z/A14 are current; absolute support life of any model of iPad is typically 4-6 years.
A newer generation CPU is no guarantee of a longer support life. Ultimately, support by newer OS versions is determined by ability to support specific features.