I think all dart frogs can benefit with some head room, even terribilis might appreciate alittle height. all thumbnails are qualified as needing vertical space. So in answer to your question, a tall tank is always better than a flatter tank.
It more or less depends on how you want your tank.For example, I have a 1.2 bicolors in a 29 gallon and they have floor space but they like to sleep in crevices in the cork bark background.I had a pair of imitators in a regular 10 that spent alot of time on the leaf litter and then used the upper parts of the tank for sleeping.
I have 3 green pumilios in a 20 vert tank and they spend alot of time searching for goodies in the leaf litter then use the broms for sleeping or messing around .So I'd say a combination of the two, both floor space and some vertical area would make most frogs happy.
When I worked for the animal laboratory we used to have the bicolor in a 80 gallon setup that was about 2ft high. The bicolor used to use the floor as well as the upper portions of the waterfall, which was very close to the top. In addition, they would often sleep in the ficus pumila that had grown up the one side wall and took over the "canopy". I used to have to look through the top to see some of the bicolor if I had to work at night because they slept in the uppermost part of the ficus.