As they are brothers, love triangle, Josen Period, different paths, Power, etc 

They are similar in terms of having main characters that are brothers and how they chose different path in life.

But there doesn't seem to be a love triangle so far, and Nokdu Flower is set in late Joseon period (1890s,) while Grand Prince is loosely based on 15th century Joseon. Basically, very different times and very different socio-political situations.

Yeah, i wouldnt even compare the two.  They are brothers, but thats the only similarity.  There isnt a love triangle here either.  Each bro has their own love story going on.  And these 2 bros love and watch out for each other even though they are on two different sides of the war.     And to add on to what @kura2ninja said,  the two bros go down different paths and cross each other time and again.  The main difference in why they went down the paths they did is 1) that one was legitimate and the other an illegitimate son  and 2)  the father raised them in a way to push/manipulate/force each of the sons to go down these different paths.     Its very interesting to watch.

Good to know.

Many thanks @kura2ninja & @JoanneSmith