I absolutely love using an iPad in my setup, I enjoy all the synth and drum programs, but especially like the various FX apps for sound design, mastering, making samples, and also enjoy using loopers.
With my Digitakt being a class compliant audio interface, I wanted to get an iPad to hookup to it full time and was in the same boat as you - which one, what size, how much drive space.
I did not want the largest screen size. I have a standard iPad for portable, general use, and the second iPad lives n a VESA arm situated right next to my Digitakt also on a VESA arm. They stay connected 100% of the time and it stays in the bedroom studio.
So my choice was between the iPad Air 4 and the smaller iPad Pro. I chose the Air 4. Storage space was important to me, my many music apps and other content hover around 40-50gb. I chose a 256gb model so I wouldnāt have to worry about having external drives etc., I also do some video editing on it and like having plenty of space. The Air 4 comes close to the performance of the iPad Pro, and I run AUM with 6-8 channels of synths, drum programs, loopers, and plenty of FX without too many issues. Some apps are resource hogs, others barely make a dent.
The most important thing for me was getting a model with USB-C so I wouldnāt be dependent on Apple accessories. I run it into/out of my Digitakt via USB and use it as a sampling source, and FX unit for the DT, and anything I run into the DTās inputs.
I have a DAW, but these days Iāve been working outside of the DAW with an Akai Force as my main unit. I can easily route that into my DAW when I wish, export project into Live etc.
My main setup is Digitone Keys (midi keyboard for Force, Digitakt, and any of the IPad apps) >into> Digitakt (all sounds go into and back out of IPad usually using AUM) >into> Akai Force.
This allows me to use the 8 Digitakt midi tracks/controls to control IPad synths, I can sample and affect the Digitone into IPad, the DT, or the Force - or all three. I can use the Keys to play the Force, the Digitakt, or any IPad apps. Itās very flexible and useful.