How to choose printer with dual extruder? I use Prusa i3 with one extruder for some years and I would like to print from one material in two colors or from different materials for one model. Therefore I'm lookig for new printer with dual extruder.

Is there some way how to measure and/or compare quality of printers with dual extruder on the market?

For example to create 3d model - ask the seller(s) to print it - and then compare? - what details to focus on?
As you suggest yourself, ordering test prints of some model is one way to do it. 

3D Hubs and MakeXYZ allows you to get your model printed by hobbyists and small businesses for a fair price. Both sites also allow you to order prints based on printer type, which I believe is what you may be looking for.

On 3D Hubs, visit on of the trend reports, and select the printer you want a sample from. Similarly, on MakeXYZ, search local makers for your desired printer.