Would it be good or bad to double major at stern or should I choose double concentrations?

This purely depends on what the other field of study is, your natural abilities and how much you incremental time you want to dedicate to your academics, and the purpose of the second field of study:

1. Finance and CS is pointless (will drag down gpa even with lots of studying, particularly if you have no previous familiarity with the material) unless you actually want to be an engineer - doing CS for the sake of marketability is not with the effort. 
2. Now finance and english, a foreign language, history, etc. for the purpose of studying material you enjoy may be pleasurable (personally worth it) and not be detrimental to your GPA. 
3. Finally, there's loads of "easier" concentrations in Stern that appear academic in nature and can be neutral of beneficial to your GPA without significant additional work (e.g. International Business, Marketing, etc.). 

Hope that helps! Imo, I would recommend #2 or #3. But double concentrations is quite common in Stern - you have enough elective room that it's senseless to not choose electives that count for another concentration - make sense? Good luck!