For a new beek, with an eye on the budget, and a requirement for only 2 or 3 hives, you would need a very special reason not to use Mod. National. IMO.
One of those reasons might be that your queen lays very well, and your brood box rapidly runs out of space.
Twenty years ago the answer would have been to add a 'super' on top of the BB to give more laying area.
This became so common that people started using brood frames and boxes which were the same size as a 'brood and a half'. Hence 14x12.
The advantage is that there is only 11 frames to go through on inspection not 22. As well as the shape advantage, of course.
Another reason might be that you wish to use Polystyrene hives, - in that case Langstroth is the current favourite, simply because of production volumes in manufacture, although I think Swienty is now making poly Nats. (available through Wynne-Jones, iirc)
The fact that 14x12 equates with 'brood and a half', should not be confused with 'Double-Brood', which could be done either with two shallow boxes or two deep boxes. (14x12 are Extra Deep)
Double-Brood is favoured by those who want to be able to split the nest so that they can 'lift and tilt' the top box and look in the middle of the nest quickly, without removing any frames. Usually to look for queen cells as a sign of imminent swarming.
Just my resume, as a new beek last year, others may see things differently.
(I did say ymmv, but Fris says I'm not allowed to say that.- forum gestapo strikes again, she's
soooo strict !
)
(makes yer feel like Max Mosley !)