First, let me say that I work for Mack Gwinn, who designed the 90-Rounder.
I've had both the 90-Rounder and the Beta C-mag, and I like both of them.
For a comparison, the 90 rounder is a bit lighter because it carries 10 less rounds, and both drums are plastic. The Beta is nice because it's a split drum that balances on both sides. However the Beta "dummy round" follower is more complex, and the whole Beta mag requires more fuss in the way of dry lube and maintenance. The MWG 90-Rounder has all the weight on one side of the gun, but it does not extend downwards any more than a 30 round GI mag does. The 90-Rounder has a clear backplate so you can see the rounds, and it is a reliable feeder, and less finicky than the Beta C-mag. As long as you can live with all the weight on one side of the gun, it's fine. Also, I think that the high-impact plastic of the 90-Rounder holds up better in the drop-test than the polycarbonate plastic of the Beta C-mag, and I've heard alot about C-mags breaking when dropped, but you never hear about a 90-rounder breaking when dropped. I do agree that the Beta C-mag has a cool look with the twin drums, and I like the look of it.
The 90-Rounder can be stored full, and was designed for that, so that's the same with both drums.
It has replaceable feed-towers too, just like the C-mag does.
In the end, with the 90-rounder being only $124, and the Beta C-mag being about twice that price, the decision really leans towards the 90-Rounder for overall cost/performance ratio.
Check the AR15.com Equipment Exchange page for ads that sell the MWG 90-Rounder.