If you ignore wind, the 250lb rider will lose at least 20W due to higher rolling resistance so he will be slower.

In general, with equal power, the lighter rider will win by a wide margin as the heavier rider will either be fat and unable to reach an aero position or tall and have higher drag.

If the heavier rider is lean and has the same power/weight ratio then the heavier rider can be faster if he has a decent aero position.