Although .ng is alot shorter .com.ng (specifically by 4 chars), they each have a role in the internet space. The .com.ng signifies company operating in/for Nigeria whereas .ng signifies a target at Nigeria and/or not at all. It can be considered another option to .com, .xyz or .trade (TLDs).
Food for thought,
Ever wonder why Google is using google.com.ng instead of .ng or etisalat.com.ng and not .ng?
The sweet part is that, both domains are from the same registry (NiRA) which means you get equal level of technical reliability and management. However, .ng is premium while .com.ng is not. Nonetheless what you are trying to do will tell explain better on what to choose.
Personally, I use both depending on what I am trying to build. Eg, my company uses [gigalayer.com.ng] because we operate and target Nigeria (https://www.gigalayer.com.ng), the choice is all in what you want to do anyway.