The 40 breeder is going to give you a lot more room to plan out a foreground, midground, and background. It will also be more forgiving of stocking larger fish, though that's not usually a concern here. The 40 gallon long will allow you to do more of a panorama style scape. The 38 tall would be good at being a headache (my opinion). Tall tanks are good for putting emphasis on top, middle, and bottom-dwelling species, but they're difficult to reach into, and you're going to need much stronger lighting to reach the bottom of the tank, which will make maintaining a carpet very difficult.
A lot of it depends on the style of tank that you prefer. In my opinion, a 40 breeder is good for Dutch style planted tanks with a large focus on variety, and the long tank will be better for a more Japanese style tank, such as the iwagumi scapes.