Transactions, tuples, and VACUUM

Morning all, bit of a general question here...

consider:

  begin;
  update a set col1 = 'p' where id = '1';
  update a set col2 = 'q' where id = '1';
  commit;

versus:

  update a set col1 = 'p', col2 = 'q' where id = '1';

Does the first case generate any more dead tuples that will need vacuuming than the second case, or are the two updates
'merged'
when the transaction is committed?  Or is the answer 'sometimes' (I would guess depending on checkpoints)?

Cheers

Matt