The 50mm will gather more light and you will be able to use a higher power at night. I would get the lowest power variable scope you can. The lower the power the brighter it will be. But 3x9 or 4-12 should be fine. I have a 6x24 and it can be hard to see clearly because of the 6 power.
And I will add, try to find rings that put the bell of your scope as low as possable, but dont let it touch the barrel.