both are easy to setup. both are getting quite amazing with edge to edge sharpness, but i would have to say that my letus appears to be slightly sharper than my friends brevis, but that could just be zoom settings.
Both are the same easiness to work with.
its hard to figure out what u want, because they are so similar. Both will have flips ( brevis soon ) they both lose approximately the same amount of light depending on which diffuser you use. But that being said, the one advantage of the brevis is the ability to swap diffusers in and out depending on what u want to achieve image wise.
Its a tough choice, but either one will make you very happy. Personally, i have a letus extreme.