Difference between first (focal length (fl.) 500 mm, ocular (o.) 10 mm) and second telescope (fl. 1000 mm, o. 20 mm)? (both 50 x) Magnification is defined as $f_{objectiv}/f_{ocular}$.
If I had the first telescope with the focal length of 500 mm, ocular 10 mm and second telescope with the focal length of 1000 mm, ocular 20 mm, is there a difference between them? Both of them has 50x magnification. 

I want to know if there is any difference in the quality of image, etc.

Thank you for all!
The larger the entrance aperture, the greater the light-collecting capability.  Now, you didn't specify that, but that does matter in your final choice.

Next,  as the comments suggest, the choice of optics depends on the cost vs. quality trades.  You want not only a given magnification but also as little coma, distortion, spherical aberration, and so on.   At a very simple (and probably unrealistic) level, the longer lens pair (1000,20) might be easier to fabricate with low aberrations.  In real life, we quickly descend into multi-element systems, or cemented doublet designs, and so on.  

I'm guessing you are talking about a Keplerian design (two positive lenses), in which case consider how much longer the physical telescope is in the 1000-20 case.