I assume you are buying Two of the same chucks, The JT6 is the largest so go with that . Why, any of the smaller tapers 33, 3, have a smaller cross section a the the junction of the chuck taper and the machine taper, smaller leads to mor flexing, not something you want when drilling. The intergrao shanks are THE best shank of all, and the premium is small, If you bend an Intergral shank chuck, most machines would be trashed.
If you think one chuck with 2 arbors lose that idea quick. To prevent seperation of arbor and chuck, clean bore, with brake clean, spirits, whatever will clean without leaving a film, warm the top of the chuck with a heat gun, put locktite on the arbor, then slam the abor end down on a anvil, or large hammer, to set the chuck, now its time to use it. The last thing you ever want is the chuck to come loose from the arbor. I'd suggest that if your lathe is large enough to have an MT-3 taper, buy a 5/8" Albrecht, a interesting point is a Albrecht 5/8 intergral shank chuck is shorter in installed lenght than a 1/2" with a arbor. If you have ever used a S&D drill bit in a 1/2 chuck and had to use a strap wrench to loosen the chuck, that is why I replaced the 1/2 chucks with 5/8" .
Your mileage may vary.