[maintech]

I can only comment re the Samsung Galaxy 12.2. It does the job well enough providing you keep the OS as Kitkat. The Lollipop upgrade has issues; for me it was the loss of music keyboard midi function, (Korg allows a link between their songbook database and the song selection on the tablet) for others the battery life has been an issue.
The Galaxy has a mini SD slot that allows almost unlimited content (64GB) for MSP. I purchased mine (refurbished) from Amazon last year for less than $380 US. Battery life is excellent and midi/bluetooth (Page-Flip Cicada) connections are now flawless. One small irritation was the need to obtain an OTG Samsung cable to link my Korg USB to the tablet, but once that was obtained it worked as promised.
The typical scanned music page can be an issue because the page proportion found in books (A4) does not fill the screen vertically, and so careful cropping is needed for a decent image, and I'm currently searching for a tablet clamp/stand that I can use to bring the tablet closer to my eyes than the existing music stand clipped to the rear of my keyboard, so that a smaller page is still readable. 
I have played around with image programs, stretching an 8 1/2 x 11 full page to the 9 x 16 proportion, and that solves some issues, but if you have a lot of music to load, that can triple the time required.