Pro 9.7".... by a HUGE margin!
Not even a contest.
CPU/RAM:
iPad Air 2 - A8X (1.5ghz), LPDDR3 RAM (2gb)
iPad 2017 - A9 (1.85ghz), LPDDR4 RAM (2gb)
iPad Pro 9.7" - A9X (2.16ghz), LPDDR4 RAM (2gb)
Screens/GPU:
iPad Air 2 - nice screen, fused, thinner, good antiglare, but lacking TrueTone
Powered by 8-core PowerVR GXA6850
iPad 2017 - thicker screen, not fused, less antiglare
Powered by 6-core PowerVR GT7600
iPad Pro 9.7" - best screen by far, thin, fused, much lower reflectivity (glare), & the incredible TrueTone technology
Powered by 12-core PowerVR Series 7XT.
Also Pencil Support & the Smart Connector are Pro only features.
This is a no-brainer!!!
Please ignore anyone that continued to recommend the 2017 iPad after you mentioned that you could get the freakin' Pro for CHEAPER (I know... only by a buck- but hey, also no taxes!), in pristine condition, w/ a free case. For serious... that's the best deal; I assure you.