Also, a PB needs one pilot. A Thunderhead need a full crew to be effective (12- 1 driver, 1 MG gunner, 10 sections of firing ports, discounting the spots for 4 more passengers).
Simply put, they fill different roles. Neither is suitable for a strict combat role once we start facing enemies with armor. (The Thunderhead will still see use then, but the PB will likely be completely obsolete.)
And speaking of which, I vote that we immediately begin to design a revised Paperback. A Paperback-MG, with the stupid Light Thaumic Drivers replaced by lighter and more efficient Magitek Machineguns. Seriously, that dual turret on the T-Tank packs more punch than what a Paperback has. They're like Light PPC's in Battletech - nominally PPC's, but you'd be better off with a laser.
I fully endorse this.
The thaumic drivers are best against somewhat armored targets, right? Why are we going after anything with armor in this tin can*? It's much better suited for mowing down infantry. Best case scenario is that the MG is top-mounted with some automatic targeting, but that probably requires higher tech (as well as sufficient roof support.) Failing that, side-mounting them where the thaumic drivers are now is quite acceptable.
*Now, it would be good as a merchant raider in its current state, but considering our likely opposition, this isn't really the case. By the time we need anything to fit that role, we can probably build a
flying one.
Armok raises a good point- making it a moddable weapon hardpoint gives us future options as well- especially if this is used as a base model for the flying version.