What is best iPad for reading sheet music?

You need to select the following points.

1. Device: iPad / iPad mini 
   • Screen size is different
     • iPad: 9.7, 10.5, 11, 12.9 inches
     • iPad mini: 7.9 inches
   • The 12.9-inch iPad is almost the same as the sheet music size
   • If you want to see the sheet music easily, the larger "iPad" is recommended
   • Please refer to the figure below for the relation between screen size and sheet music size (Blue part is the each screen size)
   • 
2. Size: 16GB - 1TB
   • It relates on how many file you can import into the device
   • As a rule of thumb, approximately 1,000 songs per 1GB
   • The smallest size (16GB, 32GB) is sufficient if you only use it for reading a sheet music.
3. Mobile line: "Wi-Fi + Cellular model" / "Wi-Fi model"
   • With the "Wi-Fi + Cellular model", you can download a sheet music on the go without a mobile router etc.
   • In normal use, "Wi-Fi model" is better solution for reading sheet music.

Please see the following link.

iPad - Compare Models - Apple