I agree that the upgrade from the anti-tank SMGs to the DAO-12 is way more dramatic than going from the engineer's shotgun to...well, a faster-shooting shotgun. Also, the PLA NOR982 and USMC M1187 do 25x8 damage, making them one-shot-one-kill weapons in close quarters. The Jackhammer at 15x8 damage is a one-hit kill only against non-armored soldiers, though if you have a full-auto shotgun chances are you won't stop firing after one shell The DAO-12, on the other hand, is basically a Saiga 12K that holds more shells (and can be reloaded mid-magazine without wasting shots). It does 12x8 damage making it a two-hit kill under the best of circumstances. For purposes of comparison, all the anti-tank SMGs do 19 damage, thus requiring 6 hits to kill (8 against armored targets). Ouch.
In other words, unless you NEVER play anti-tank (or are willing to be a masochist with a glorified machine pistol when you do), the DAO-12 is simply the better unlock. Even if you don't intend on going after tanks as an AT or repairing stuff as an Engineer, the AT guy will perform better at straight-up combat considering that he has body armor and the Engineer doesn't. (But shame on you for not using your SRAW or wrench, lol.)