2NL and 5NL being the same is a myth, at least on FTP.
I won at 2NL from the start (although with nice help from the Lady) and at the end I was winning at 10bb/100 clip without much effort while 5NL is a different story (Lady hates me since I moved up but hey, women right ).
I suppose it may have been true once and of course if a player is really good then he might not really notice much of a difference.
5NL generally has more calling stations and more diverse player pool. You will encounter a handful of decent MSS players and hopefully learn how to combat them (hint: their strategy is very static except for tendency for 3bet exploitation) also some of the players (like us) have started from zero and are on their way up so by default the player pool has to be a bit tougher.
I find it challenging and believe it will allow me to sharpen my skill set before I move up eventually, but it is frustrating at times (depending on the composition of players on a table).
If you get into a downswing, look me up, everybody runs like God vs me