10x42 exit pupil is 4.2 while 12x42 is 3.5
That's a pretty significant difference in terms of the amount of light that reaches your eye. Generally, the larger the exit pupil, the better the viewing experience (all things being equal).
The quality of the optical lenses is exactly the same between the two models (i'm guessing) because they sport the same name and are likely made to the same optical specs (i.e lens coatings, # of lenses, focus knobs are the same, etc) however this does not tell the full story of what you will see when comparing these two binos.
I have owned the C1 10x42's and firmly believe they are one of the best sub-$500 purchases you can make when it comes to 10x42's. I have not tried the 12x42 version but if I were you and were leaning towards a 12x I would try the 12x50 C3. The larger exit pupil alone should make the experience more enjoyable.
Be warned though, when you step up in size the objective diameter of cheaper (sub $1000) bino's, the small issues that are evident on 42mm models become more noticeable.
