Hi olio. They both have their advantages/disadvantages. I'll try to make a brief list about it

Modo:
- speed, both for setup and render time. But beware as you push modo further towards quality (and further away from the default settings) it will slow down greatly
- easy shader system, but will actually become a problem on complex materials
- interactive preview render makes light/material setup very easy
- has some lighting artifact problems when using blurry reflections
- excellent render pass system
- good and fast displacement
- does not produce accurate color match to the system color picker, unless you make adjustments to gamma and also calculate the hue difference to get your correct output color. Its a pain in the ass if you need to accurately match Pantone standards. I've not heard of any improvements to this in 401.
- many reports of poor network rendering implements
- many complaints of banded images on interior GI renders
- lack of "per object" and "per material" inclusion and exclusion lists
- lack of good procedural textures

Brazil:
- nodal shading system, complicated but extremely powerful. You can use any shader or texture to drive the effect of any channel in the engine.
- accessible control of any single parameter in the engine. Once you learn Brazil then all other engines look like kiddie toys. This can also be a negative for new users, but the UI can be rolled up into a "simple" mode to make setup extremely easy
- excellent antialiasing routines and controls, extremely important for CAD models. I know that modo 401 will improve this area, but I've not seen the results of it yet. In my experience, modo (201-302) failed horribly in this area. A couple of weeks ago someone also posted a comparison of AA in Brazil vs. Vray and Brazil was the clear winner
- excellent routines and controls for render cache
- shadow mapping
- photon mapping
- artificial lighting and shading effects
- not as fast as Modo, but less true as modo approaches same quality
- no network rendering yet, but it's in discussions now. Also it is reportedly the first engine actually running on the Caustic One raytracing card. : ) !!!
- takes advantage of all Rhino RDK features such as edge softening, shut lining, curve piping,
geometric displacement, etc (micro-poly displacement will be coming soon)
- no fiber/hair/grass generation
- no advance animation (rhino shortcoming)
- you don't have to leave Rhino
- can easily add occlusion effects to "ground" objects so it's very east to avoid "floaters" when undersampling the render cache. Also this is very good for enhancing geometric detail in your models

It's hard to recommend one or the other, as your needs may not be the same as mine. But I stay in Rhino whenever possible as it makes my project time much shorter. But I'm mostly doing product shots. If you need to do something more than that, I would recommend modo as a good choice. It is very easy and fast to get nice results. And when it behaves it actually makes rendering a pretty fun experience

jonah