When using the bigger tablet from Apple, I always pair it with a keyboard case. This makes the writing as easy as opening the clamshell and getting right to it. I can detach the iPad Air 2 from the keyboard unit for the online research phase of the work and use it alone which is much more comfortable.
My preference is going as light and portable as possible, so recently I considered going smaller with the iPad mini 3. I do light work on the iPhone 6 Plus when I have no tablet with me, but there's no way I'd head out for a long work day with just the phone.
I wasn't sure if it would be good enough given the smaller display but decided to get the iPad mini 3 and give it a try as my daily work system. I picked one up from Apple (gold, 64GB, LTE) and have been using it every day since the purchase. I paired it with the Jorno Folding Keyboard for the heavy amount of writing I do daily, and the pair is as productive as the bigger iPad and a keyboard case.
The primary benefit of the iPad mini 3 over the iPad Air 2 is portability. The mini fits in a pocket or in the smallest of bags for carrying around town.
I use the tablet in hand for hours a day doing online work that doesn't involve writing. The iPad Air 2 is pretty light but the iPad mini 3 is much more comfortable to hold for long periods. The difference is much more than the weight of each tablet would indicate (0.2lbs). Just a few ounces combined with the better balance of the smaller iPad is so much easier on the hand that it makes up for the smaller screen.
Unfolding the Jorno keyboard gets me in writing mode in seconds. The keyboard is as wide as those attached to cases for the iPad Air 2, so I lose nothing in the process. I write 1,000+ words on a typical day with this smaller system so I'm losing nothing by going smaller.
The iPad mini 3 is better than my iPad Air 2 in one significant way. I learned from my mistake with the purchase of my bigger iPad and got 4G/LTE on the smaller one. This greatly facilitates using this little writing system anywhere I happen to be.
The iPad mini 3 is not only better than the bigger tablet for work, I also find that to be the case at home. It's easy to keep it at hand for quick online excursions. I read a lot at home and the iPad mini 3 is the best way to do that as it is the perfect size. Plus, as it is so comfortable to hold I'm finding it great for gaming.
Such a small solution for work is not for everyone, but it is working very well for me. I have never used a system this small that is as functional as the iPad mini 3, and I'd make the switch again in a heartbeat.