I think everybody should see a recent Alex Tabarrok's class on the effect of geography on institutions and long term economic growth. It is here, and it is superb!
I would add three things:
1) It seems that the North American colonies had higher religious freedom than the South American colonies. This was important for the development of freedom of expression, freedom of press, etc., which are highly correlated with economic growth.
2) Probably the English and the Spanish "exported" different values to their colonies. The British might have exported decentralization, and the Spanish a more centralized way of management.
3) The Spanish empire has been called an "empire of conquest," while the English an "empire of commerce." This might have to do with geography and endowments (as Alex says), but also to the fact that the Spanish Crown was stronger, it was richer, than the English crown during the early colonial times.
These points are based on my reading of Eliott's Empires of the Atlantic World: Britain and Spain in America 1492 - 1830.
In any case, Alex's class is great.