What information do I need to select a circuit breaker?
Running 20amp for your workbench is a good idea (and be sure to use 12/2 wire), but overkill for a computer circuit: most residential circuits are 15A and are run on 14/2. 

You don't have any choice in the breaker: Siemens panels only accept Siemens breakers, and StabLok panels only accept StabLok breakers (and of course there are several other brands). The bus design in each panel is different, and so other brands of breakers won't fit. This isn't the full truth: there are other companies that make breakers that are compatible with Siemens panels, but they'll be marked that way. Just realize there are different designs of panels with different designs for how the breakers physically connect in, and they're not all cross-compatible. 



There are some known problems with the design of StabLok panels (which are made by Schneider Electric, and sold as Federal Pacific, Federal Pioneer, StabLok). InspectAPedia has some pretty dire warnings against using FPE StabLok panels - I was actually not aware of the extent of problems until just now. According to the article, it's actually a fire hazard as they can come loose, and that's aside from the apparent defects in the breakers themselves (they can fail to trip, and even fail to shut off when you manually turn them off). 

I personally am not a huge fan of StabLok, the connection is just not as solid as you get with, eg, SquareD panels, and most StabLok panels are ugly for that reason: all the breakers are skewed in random directions.