The mini is a great portable and super, super light iPad with some great horse power, and then you have the much more powerful beast that is the iPad pro (along with that giant screen.) The mini is super portable while the Pro is not as much.
Personally I have the mini 4, and it's great and all, but I feel since I will have the iPad Pro and a 6s Plus, I'll have no real use for the mini. I'll still wait and see after trying out the Pro for a while though.
If the Pro is enough, I'll sell my mini 4 to go towards buying a Retina MacBook gen 2. I need a laptop for basic work and stuff.
***sorry I went off topic.
To answer your question, I don't think you need both. But since the price of the Pro is so much I'd research to see which one would be more suited for your needs. So look at some reviews of the mini, try both out in an Aplle store, and then decide which one, (and hey, you may decide you want both.) In the end, none of us can really say what you should do. Either way it's your money, and if you are okay with, and can afford to pay for both... Do that.
Either way you win.

Sorry if I wasn't much help (or my opinion is weird and contradicts itself. lol)
Kal.