For JUST the myDMX Go app, my guess is 16Gb is sufficient, so if it's the only use, get the minimum. I use my iPad as simple playlist player or emergency backup, so I put 128Gb in so it can hold a bit of music as well.
I agree with Steve - but iPad is one of those you can't upgrade later - so get the biggest you can afford (you'll thank yourself later). You will end up using it for other stuff too. I went with 128 as well.