In both cases you want 15V input and 2.5V output. This is a way too high a drop and even though your current is only 50mA, will result in a relatively high dissipation. Best would be to use a pre-regulator and drop the 15V to 9V or so and then go in for the final regulation to 2.5V. ( LM 317 is a good choice for this as you can arrange the resistors for exact 2.5V )With a pre-regulation your ripple rejection will also benefit.

Raghu