[Let'smoke]

The other design I've seen is also quite good, and seemed to have a bit more surface area which could arguably make it more "effective."
Carbon for carbon, the one with more surface area would also probably run out faster. It would however, allow for a little more airflow, I believe.

Everything's tit for tat in the growing game. I gave nugglord +rep for this design because of how easy it would be to change out the carbon. I could imagine someone really getting into trouble because they realize too late in their grow they need to change their carbon and are too busy. Plus I'd freak for every second there wasn't a filter on my grow.
That's why if I went with the other design I'd build 2, that way I could swap it out in a snap and refill the first at my leisure.

These are all just my opinions of course.