A10X is going to be slow in the near future given the exponentially rising speed of ARM processors. RAM deals with background activities, whereas processors deals with the processing speed. As such, with the 2 generations of improvement on A12 over A10X, you are going to get a much faster processing speed on iPad Air 2019 than iPad Pro 2017 even with 1GB less of RAM and the absence of 4 speakers and pro motion technology.
When ProMotion at 120 Hz, intensive background activities, or sound quality is a priority for you over processing speed, GIVEN THAT you want a budget device, then you may consider buying an iPad Pro 2017. However, if budget is not your concern, why not purchasing an iPad Pro 2018 which is the most future-proofing device in the market?