[rcxdude]

They're both correct in that both need to be true for lift to occur. If there is a force upwards on the wing then there needs to be an opposing force, and this necessarily must come from a difference in air pressure between the top and the bottom of the wing. Also, for the force upwards to exist, there must be an acceleration of the air around the wing downwards. It's also true that for a pressure difference between the top and bottom of the wing to exist, the air must accelerate downwards and vice-versa. Similarly, the low-pressure air on the top of the wing must be going faster, and faster air must be lower pressure (it is, however, not true that the air travelling over the top of the wing must take the same time to pass the wing and the air travelling over the bottom. This is in fact usually false).

These are all fairly straightforward consequences of the basic rules which air must follow when flowing. The wing produces lift because the shape of it means that all the consequences of the above are the only ones which follow all the rules, much like a sudoku or crossword. The details of actually solving the full puzzle (in way which tells you how much lift and drag you get) turn out to be fiendishly complex, but the basic reasoning is not too hard to understand, apart from the fact that people get confused by the fact that you can explain it 'simply' in multiple different ways by focusing on only one part of the whole crossword.