[laba]

If you use standard A4 paper, (which I assume most of Europe does), then 4:3 is the best fit as this is almost the same aspect ratio of the A paper sizes. Ideally it should be somewhere between 4:3 and 3:2, but I don't think the latter exists as a screen format.

If you use it in landscape mode, 4:3 is also still the best fit, as the aspect ratio of the A paper standard is the same regardless of paper size (i.e two A4 pages side by side has the same aspect ratio as a single A4 in landscape orientation). So a 4:3 screen in landscape would fit two A4's much closer than a 16:9. 
I don't think you can get a 4:3 screen much bigger than 14 inches (iPad aside), and using two page mode on a 14 inch screen is kind of to small, at least for my eyes

As an example I have a 12 inch 4:3 display and a 13.3 inch 16:9 display. In portrait mode/single page the width of the 4:3 display is about 1 cm wider than the 16:9 display, making the size of the sheet music bigger on the 12 inch than on the 13.3 inch. But the widescreen does show a little bit of the next page, so if that is what you want go with that.

Of course, if you use custom scans with custom aspect ratio all the above is invalid