If you're running a fairly grunty A14/15, particularly in competition, then the Toyoyoyo box is realistically the only way to go & as Burt said, they are almost unbreakable, but they are a bit big, heavy & power hungry for a modest, sports oriented 1200 street engine.

In this case, the 56A five speed could easily be the better choice, .... if you can find one, & if you can afford it when you do.
For a street 1200 with a bit more pep, the 60 series boxes are a better idea & they seem to be a little more common. [like two or three needles in the haystack instead of just one.]
These smaller gearboxes seem to be able to handle the lower torque output from the short stroke engines OK, but hot, longer stroke A14/15's just give them grief.

I have been rather fortunate in recently purchasing two of the 56A 'option one' competition gearboxes [F5C56A] & if that works out the way I want it to, my regular version of the 56A five speed [F5W56A] may well be coming onto the market, but don't hold your breath waiting, it won't be for a while if it does.

Does anyone know of a smaller & lighter, but still readily available 5 speed that can be adapted to the A series for use behind the smaller engines?