If you stand close to your rig, an angled cabinet can be easier to hear, as 2 of the speakers are more directed towards your ears.
Since many bands also mike the cabinets, the mains get the same sound from either type.
A straght 4x12 cabinet can be somewhat directional for audience members who walk across the direct path to the speakers. An angled cabinet spreads the sound around a bit more, and is a little less directional in nature.
I use angled 2x12 cabinets, one on each side of the stage.
Ed