I usually play bots when I want to try some moves out. When I'm looking for more of a challenge I will play humans.
Interestingly when I play bots I am less anxious and don't really care if I lose the game.
On the other hand when playing people I get more anxious and really hate losing! Therefore my game suffers.
Computers are worse than actual players at their rating... Like how I can easily beat the 800 rating Beth, but I can't beat an actual player at 800...