So went through the same questions a few years back when looking for a solution for a third function for our NH... The 1st thing to consider IMHO is how & what are you using the third function for... if it is for a grapple or similar type attachment where you are going to be frequently switching back & forth many / multiple times to use the function (i.e. grapple, post puller, etc), then I would never want a manual switch like the kit you linked above.
I agree the install is pretty simpler / straight forward but at $600 (= or-) for the kit you are very close to purchasing a electric diverter valve which will allow much quicker & "covenant" operation of the 3rd function diverter; i.e. never taking you hand off the loader control.
When I purchased ours, I looked at it as a long term investment & for near the same money & a little extra install time; & after a lot of usage I would never not want the system we have.
Again just my opinion, nothing against the product you linked (I looked at a similar solution back then) & also looked at a true 3rd function... but decided on the 3rd function diverter with momentary push button control.