Well said, Rob! Not much I can add to that except to say that there are short Triumph nibs and the regular ones.....and of the two I prefer the regular type. Of the regular type of Triumph nibs, there are two sub-types: the earlier shorter, fatter nibs from the 1940's and the later narrower, longer ones on the Snorkels,etc. I personally prefer the earlier ones, just on the basis of aesthetics.
As Rob said, the plunger-fillers are much harder and more $$ to fix than the lever-filler types, so this is something to consider when you are looking to buy one. If you can find a working or restored plunger-filler, go for it! They have a large ink capacity.
I have a carmine-striped one with a wide gold band that is a dream (stiff nib aside

).
If you are having trouble finding a working plunger-filler, then go for a Touchdown filler or Snorkel.