It really depends on each airline. In "my" outfit pilots are asked which equipment they prefer (A320 or 737), however typerating slots are distributed on fleet demands, if someone isn't happy with the type he gets a shot at he can take the chance and wait for a chance on the other one. First bids go to in-house cadets where the typerating is part of their initial license training (MPL).
Currently longhaul flying is possible for pilots on the A320 only as the longhaul fleet is A330 and flown in mixed fleet flying with the small one. However our company council currently works on a scheme to give boeing pilots the possibility to switch fleets. In the future the same is planned however for the boeing fleet anyway, mixed fleet flying 737 and 787s.
Every bid is done on seniority of course.