Re: digital photo frames
I have both an iPad and an inexpensive digital photo frame. The nice things about the photo frame are the cost is much lower so I am less likely to cry when the inevitable studio accident occurs, it will load photos from a USB, iPad does not have a USB port, and it will display longer without going to sleep.
The iPad has the advantage of also playing music, surfing the web, and having image manipulation abilities. iPad also has touch screen zoom which can be nice.
My photo frame is about the same screen size as an iPad, which I actually find a bit small for using as a photo reference. This is perhaps the biggest advantage of digital photo frames, you can buy them in larger sizes. You can get a cable to connect your iPad to a large flat screen tv, but most artist I know do not have one of those in studio.
If you can get that 15" digital photo frame for less than the cost of an iPad, that is the route I would take. Unless you really want an iPad for other uses.