I have a rooted Galaxy S5 Neo running stock 5.1.1.
I want to do a factory reset without removing root. Should I use the stock factory reset or TWRP?

If I use TWRP do I select 'Internal Storage' as well as 'System' under 'Advanced Wipe'? Or will erasing 'System' remove the ROM as well?

If I use the stock factory reset will I lose root?