Because of what you will be ending up with after your latest project is completed I would say the larger QT.
You'll have a pond or two outside. You'll have an indoor pond in the new sunroom. Having a single QT should work well for the three purposes you may be planning for - Hospital for the occasion it may be needed, Quarantine for new purchases, Winter housing for small fish from outside that for some reason don't go to the indoor pond.
I also see it as a possibility that you could use this for all of the fish from one of your ponds if there were some emergency, like if you had to make a two week long repair to one of the ponds.
Plannning your fish purchases, you know, like limiting them

so that QT is ready and available should be OK and easy enough to coordinate. Having 1800g to QT in should make the job easier, a bit of space for the fish to relax and move around in.
12 x 5 x 4? I don't see 12' sides as a big deal, but steel is cool. Remember it is just a tank. You can put a brace across the top. The brace can help hold the net. Plannning for good bottom drains is the tricky part for me.
Mickey the windowman