I would go with high searches. You also get more traffic off "broad" searches, which is when you are indexed temporarily off broad keywords (e.g. being indexed for "mobility scooters for sale" when your domain is mobilityscooters.com).
But I think the nature of the keyword and competition are just as important as how many searches it receives and the payout. Keywords related to product and services do better as mini-sites, especially if you can provide a lot of original content related to the subject.
And life is easier when there aren't 1M domains that are also trying to be ranked on the first page of Google for that term.