I went with the 7U Intellijel case. More expensive but is all aluminum in the chassis and comes with a locking lid and a carrying handle.
But for your FIRST case and a choice between the Mantis and Rack Brute, I'd go with the Mantis... especially if it's not meant to leave the house. The Mantis also comes with adjustable legs. I find that to be important as you'll be spending hours looking down at your rack. I don't like giving up a rackspace for power supplies.
The only downside to the Mantis is that it isn't stackable like the Rackbrute. But my own personal taste is that I don't like my rig vertical or in a "wall" configuration. I like table-top views. Those may be considerations for you as well.