IMHO the iPad and Pro are very different products even though they share the same OS. One is for the entry level "value" market, the other for the traditional Apple customer who is attracted to Apple because of it's product's build materials. With the iPad Apple is reaching down to the Android market in the same way it tried to reach down to the entry PC market with the Mac mini. There is nothing wrong with the iPad, it' just isn't refined like the Pro. Apple built it the least expensive way it could and it's quite apparent. That isn't a "ding" in the iPad, it's just a description of what the product is...or is not.
So my suggestion would be to go to an Apple Store or other store that has working models of both the iPad and Pro on display, and not encumbered by heavy anti-theft mechanisms. Feel them out, stare down the screens, hold them like you do for a few minutes, and imagine using it now, next year, and for however long you think you'll keep it. The iPad will age quicker because it has an A9 chip (just one generation up from the A8 in your Mini4) and 1/2 as much RAM as the Pro -- something to consider if it's a longterm purchase.
Another consideration is we are going into the holiday season and already we are starting to see good occasional discounting on the Pros, esp. the 10.5 model. That one is regularly $50 off. I suspect it might dip to $75-100 off by October. So if you like the Pro but not the price it might be worth waiting another month or if you have a CC with price protection buy within whatever window gives you that protection for the holiday season.
