Is MSc the same as MSc (Eng)? I have a master's degree in the UK, titled MSc Advanced Materials Engineering.

I would like to know if it would be correct to put MSc (Eng) as a post-nominal?

There is no clear guidance. By using just MSc in a post-nominal it is of course not implicit that the qualification is in an engineering subject. I would like to make that distinction if it is permitted to do so.
You hold an MSc, not an MSc (Eng), so use the former. 

If you want the reader to know you have an engineering degree, then write MSc Advanced Materials Engineering, perhaps even MSc, Engineering dept., Uni X for something shorter.

Note that MEng also exists, so MSc (Eng) may baffle some.