Speed wise the 2 new ones should be similar pegging as they both have the A7 in them.. That will e much faster than the mini.
The current mini manages well enough wit asks but I do sense that the creep of advance and complexity is starting to tax it at times. So buyin one now means that you be hitting its limit much quicker than he new ones. For that reason alone, I would go for a new one.
Difference hen falls down to size and what you want to use it for. The mini is handy if carried about. If it sitting in the lounge all the time, them the larger screen is better I reckon. I have an original iPad and a mini. The mini is faster, but I like the larger screen of the original.
The original is getting past it, and am considering a new iPad. At the moment i thinking of going for the larger iPad air at the moment as I don't need e portability.