As an F/O, you're locked on the type you're hired on to till upgrade. Company can unlock you if they want to. Classics run under 121.483/521 so up to 12 hours in the seat but you should get about half the landings. On 400s you can only be in the seat 8 but you may only get 1 or 2 landings a month, some F/Os have to hit the sim for landings. But the rest areas and galley are better except on the converted freighters. The classics do more military work and were doing most of the South America trips but the 400s go to SA a lot now too. And the 400s do AMC trips too.