although some IDE's have an option to automatically convert tabs to spaces for you.
This. I always turn this on in IDEs I use.
tabs produce inconsistent looking code, as the tabs will look different to another person unless they're running the same editor and the same font face/size.
As far as style goes, I think you should adhere to the style of the project you're working on. That applies to the use of braces, tabs, naming conventions and layout.
m_ was introduced by MFC. It's a standard for MFC classes. If you use MFC, use m_ as a prefix. It's consistent.
I've always used 0 instead of NULL for C++, but I can see where nullptr is a better choice (it isn't an int), so I'll be using that where available.