I have a slightly different take on this. Catch and take are interchangeable, both in AmE and BrE (I have caught and taken quite a few buses both in the US and in the UK).
On the other hand, there is a different "flavor" to the two words, as the other answerers are saying. Hellion puts the difference very well: if there's some chance of missing your bus, then catch is the verb used; if not, then take.
The reason that take and catch are interchangeable is because, as I'm sure Hellion will agree, there is always a chance to miss the bus. You can be standing there 15 minutes early, have the bus show up, have one person get off, and have the driver let the person in front of you on and then say "sorry, all full, catch the next one."
Catch conveys the meaning of buses always attempting to leave one behind (which as everyone knows they are) and therefore having be caught to be used. Take is more matter-of-fact; it simply means using the bus as the means of travel. So, as I see it, the question of whether you use catch or take is simply a question of whether or not you want to convey a sense of identification with the stress shared by bus travellers the world over who are depending on a bus as their means of transportation.