A majority of people I've talked to agree that threaded cans have less POI shift with greater repeatability, but a majority of those people also own muzzle device attached cans
. If you want to be a perfectionist and absolutely reduce the delta between POA and POI and increase the repeatability, then go for direct thread. On the other hand, if like many, you want to quickly and easily move your cans around between your rifles, then consider otherwise.
It is also possible that many local class III dealers will prefer to sell what that have in inventory or what they have best known discounts for. Shop around, call other dealers. Call Silencer Shop, they are kind of a "national" dealer, though you still need a local to do the final transfer.
Get more input before making your decision. BTW, my next suppressor will most likely be a break attached TB, this is so I can move it between .308WIN, 7.62 NATO and .338LM.