I'd say, try to go back to sleep. At first, I was saying "wow! maybe I just need 15 minutes naps" and started calculating, how much time this will save me in 30 years. Then I would crash.
But you can try. Aim for your usual nap length, but if you woke up early, wake up and pay attention to your state. Is your waking period after the 15 min. nap is different from your usual nap. Are you alert? Are you feeling unusual urge to sleep in the middle of waking period? If you see that your performance suffers, stick with the longer naps. If you are ok, aim for long, but let yourself to get up if you wake up early. If you are consistantly waking up after 15 mins. and feel fine, try to make it your standard nap. But don't be surprised if an oversleep happens.
Also don't confuse real wake up, with a just-in-case wake up that your subconscious does sometimes to check that you are safe in this strange thing you are doing. After a real wake up it is somewhat hard to go back to sleep.