If you are buying an iPad as a laptop replacement it's worth buying an air or pro. I you draw or write on it a lot, the pro is worth the upgrade to get the faster screen refresh rate and a better writing experience.
If you have a laptop, the mini is the best iPad. Being able to hold it in one hand makes it much more useful. If you are going to work at a table, just use your laptop and get more done faster with the better OS. A 256GB iPad Pro with magic keyboard costs more than a 256GB MBA and weighs nearly as much.