Other than the fact that it's 12 lbs lighter, is there any reason to choose the 33lb bar to lift rather than the 45 lb bar?
I'm assuming you're talking about Olympic bar types. The differences in weight 'seem' to be related to the width of the bar, the 33lb sometimes being called a woman's bar since it's diameter is smaller. The larger the diameter of the bar, the more it works the grip/forearm muscles and helps activate the CNS (central nervous system) in some cases (I'm not sure if this is scientifically proven) helping the person lift a little heavier.
If you're lifting a certain weight for your program, the weight of the bar needs to be taken into consideration to determine total weight BUT the weight itself is not important. What does matter (as stated above) is the width, the stiffness (helps with some lifts hurts others BUT only for high end lifters) and the ability for the bar to hold some max weight.