I think now is a great time to upgrade an original mini. The Air 2 and mini 4 both have 2GB of ram, and I think they'll last a while. Either would be a huge upgrade over the original (non-retina) mini. If you're thinking of the Air 2, there have been some good sales and you might be able to find a deal on Black Friday. If you're thinking of the mini 4, it was upgraded recently, and so is less likely to be upgraded soon.
As to whether you should upgrade to the Air 2 or the mini 4, that's a matter of personal preference, only you can decide. Do you like the form factor of the original mini? Do you find yourself carrying it around with you in a jacket pocket or purse? The mini 4 is even a bit slimmer and lighter. Or do you find yourself wishing it was bigger, that the text wasn't so small, that movies were bigger? Then consider the Air 2.
