Google iPad pro comparison. The newer model, I believe has upgraded processor (A9x vs A10x). Newer model may also have twice the ram. Not sure you will notice big difference in everyday use. Apple quotes 30% faster.
The better decision is screen size. You should go to B.B. and hold them. IMO, the 12.9 is too large for portability in the field (to carry around and use mobile).
If going to use sitting around, larger pad might be fine. I didn’t look at screen quality for the larger screen. I would assume that apps scale to the larger screen, and I don’t know if that will pixelate on the 12.9. I haven’t noticed issue on the 10.5 screen.