Herbie/bean to keep it simple are generally in reference to the number of drain holes it will have (per overflow)
A primary and secondary for herbie and add an additional emergency to make it a bean (with both having low/no noise as the goal)
Depending on the placement of the tank, an external overflow takes 0 real estate in the tank and plumbing the overflow on the side (peninsula style) or back is much easier than under the tank when factoring the sump and ends up adding more water volume which is a huge plus in my opinion.
Ultimately though regardless of which overflow style you choose, I would strongly suggest 3 holes minimum (primary + secondary + emergency drains) and then triple check placement of said aquarium before picking corner/center/external overflows + door clearance.
One of the worst things that could ever happen is you order your dream aquarium and you can't get it through the door or even access the plumbing due to the external overflow/placement of the aquarium.