The way I like to think of it, the longer the cooldown of a summoner spell is, the more effective PotM is, since it's a scaling reduction rather than flat reduction. It also works on both of your summoners, and you can pick up the Flash mastery on the way down and get it to a reasonable CD when you take both.
That said, masteries are largely up to preference. I just don't really like Burning Embers because I typically save Ignite for after I've already blown all my cooldowns and just want to make sure I do enough damage for a kill without them being able to drink a potion. The extra 10 AP when it's on cooldown 1) isn't very much bonus damage on most skills, and 2) tends not to come into play when I most often use Ignite. I'd rather have it on a shorter cooldown so I could use it more often like this than get some free (but practically unnoticeable) AP.