Hi Leon, Definitely a tough choice. No matter who the supplier, the 12" GSO dob is an excellent upgrade for the estimated budget. The apparent difference between the suppliers could be levelled out with a bit of haggling. However, as you correctly identified, the 12" dob is not entirely transport friendly.
I've been blessed, where I work, to have a company supplied vehicle which easily accommodates the 1.5m length.

Unfortunately I won't be in this location forever, with this vehicle, and I will have to return to the family vehicle one day. So last week, I checked on the specs of the family vehicle and found out that unless I take 50mm out of the OTA it's not going to fit.

My only way around this deficiency and keep the OTA inside the vehicle, assuming we keep the current vehicle, is a little mod I'd prefer to keep the Qld RTA unaware of.

If I remove all together the fold-down rear seats of the Pajero there will be enough room.

On the other-hand I could upgrade to a 16" truss dob, keep the un-modified Pajero, still travel with the scope and continue to feed my aperture habit.
In summary, if you can hold out and save a little more for the truss dob, you're need for more aperture and the ability to travel with it, can be satisfied for not much more than what you're prepared to spend now!
Good Luck,
Graeme