VAS

To explain, Stansted is one of those airports, like Atlanta, which have a central terminal and remote "satellites" out on the airfield. Ryanair use the south satellite. From the aircraft you go up steps to the top level, then walk a distance to a whole string of escalators that take you down, down and down again into the basement where there is a shuttle train, every few minutes, to the main terminal. This is where you go through immigration, move on to collect your bags, clear customs, and are then out into the main terminal. The buses are outside the exit doors.

I really only mention the distance because the bags have to make the same journey by truck after being offloaded from the aircraft and that seems to govern how long they take to come through. Ryanair are a very cost-conscious airline and if several aircraft have arrived just beforehand, the ground crew will just be working them all in turn.

A rental car to Heathrow would probably be slower. The bus is non-stop, from terminal door to terminal door; it's just the question of waiting for the next departure. It would take a good while to rent at Stansted, but will take much longer to drop the car at Heathrow, where all the rental depots are outside the airport (yes, I'm into miles again); they transfer you, slowly, by van shuttle. If I was already committed and a visitor from overseas not familiar with the M25 motorway round London I would rely on the bus for this one.