Why one or the other?
because it suits members of certain political parties to pretend that we cannot afford both.
they get be seen to be suggesting that their core voters are being hard done by as a result of giving help to foreigners.
All the figures I've seen for clean up/pumping costs have been trivial in the scheme of things. We probably spent more on bombs in the first day of the last Iraq war than has been spent on sorting out the floods.