There are a number of factors I would consider:
- the distance you are away from the music stand. You play keyboards so the iPad might be further away if you add a stack of keyboards in front of you. (For others reading this, if you are a guitarist who plays standing, then you can wander up closer if you need to read/check something as you are playing, but if you are seated holding a tuba, you will be further away so a bigger iPad would be better to see all of it clearly.)

- if you are only reading chord charts from iReal b, check with a song with quite a few systems (lines) of chords. With more systems, the size of the chords will get smaller. Test with say, Brazil (from the Jazz standards 1300 playlist.)

- if you are also reading notation charts (outside iReal b) then you will need to check you can still read it clearly at the appropriate distance. Depending on the layout, size of staves, cropping, page size, margins, zoom levels, resolution (etc.) of the charts, you will probably find the larger iPad better for this. If you only play acoustic piano, the iPad is very close to you, so the mini might work, even for notation.

- also consider other purposes you will be using it for, like typing (directly on the tablet), on the mini, the keyboard will be smaller (which would be either better or worse, depending on the size of your hands/fingers and how you type.)

- the orientation you want to use it in (portrait or landscape.) Most would use portrait, but if you had problems with just a few charts (notation or not), then you could rotate to landscape to see everything larger (meaning the mini might work most of the time and when it does not, rotate.) Doing this you would need to make sure you can swipe for the bottom part of the page.

- cost, I usually take a backup device of some kind (as Bob mentioned also) so you could eventually get another [something], meaning make a choice now for the best you think would work, keeping in mind eventually getting another (perhaps a used one) of what could be the other size as an option for the future.

- the iPad 2 has the older 30-pin connector. This would be a consideration if you (or your friends) have older devices that you need to use for midi, audio, video (etc.) The new lightning connector is in the mini iPad, so any adapters you purchase for it will be useful on any other iOS devices you get in the future. (Being a keyboard player you might want to use other apps needing connections - iOS midi synths, send midi commands out to sound modules, record via USB interfaces, use iOS effects apps for your keyboard sounds etc.)