Q.pot sizes
If you have 2 offers for a brugmansia plant, one in a 4.5 inch pot or one in a one gal pot, which is the wisest purchase?
Brugmansia should be planted in large containers, at least two feet in diameter, to allow the plant enough root foundation for it to branch out - they grow super fast. With this in mind, I think your decision boils down to this - which of the two brugmansia plants is healthier? Regardless of which plant you choose, it will need to be eventually repotted, with more immediacy for repotting given to the 4.5 inch pot because that pot is really small.
For guidelines on choosing healthy plants, please visit the following link:
https://www.gardeningknowhow.com/garden-how-to/shop/choosing-healthy-plants.htm
For more information on the care of brugmansias, please visit the following link:
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm