Honestly, I don't know too much about cases. A whole bunch of airflow, cfm, and other crap I don't know. My judgment is based on what I can see.
K7B has fewer 5.25" drive bays, less cage in the HDD cage, and an extra fan. It's also got a top-mounted PSU and only a hole for cable management instead of an entirely open end. That open bottom on the K58 just looks like cable management would be much easier, especially if you've got a tough time of getting cables to reach. For instance, I can't route my CPU cable back there and if it wasn't completely open, I'd be even more SOL.
If I were you, I'd keep my $100 and save up a little more for something like an SSD or SB CPU.