Are you planning to shoot indoors or outdoors? Umbrellas are difficult to use outdoors as they act like a sail and it only takes a slight breeze to send your umbrella - and flashgun crashing down. You either need a serious stand weighted down, or someone holding the umbrella. Even so I'd go with the smaller size for shooting outdoor.
A larger umbrella gives a softer light - at the same distance. However it's a apparent size of the lightsource as seen from the subject that defines the softness/hardness (hence the character of the light). e.g. the sun and moon have the same apparent size, viewed from the Earth. The sun is much bigger, but at the same time further away.
Therefore a 33" umbrella at a distance of 2 meters should act very similar to a 42" umbrella at 2.5 meters. However:
You will receive less light from the further away brolly (i.e. bigger brollies need more powerful flashes in general). Likewise you need a bigger stand etc.
Having the light further away makes it easier to control flare (by ensuring the lightsource is not in frame) and gives the subject more room to move.
A further away lightsource gives more even lighting. Again taking the sun as an example the near and far sides of a 1 meter object are effectively at the same distance from the sun and receive the same amount of light. However if you place a lightsource 1m from the object then the far side is double the distance from the near side and receives 1/4 the light.
Finally, the difference between 33" and 42" is not that great, and 42" isn't that big in the scheme of things, so unless I was putting together a kit that was designed to be ultra mobile I'd go for the 42".