I think there's a rules oversight on the choking side of things; while a creature can hold it's breath for a minimum of 30 seconds (if it has a negative con modifier, which hardly ever comes up), the next paragraph of that rule says: "When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round)." (emphasis mine) So I'd say that there's a difference between holding your breath, and being actively strangled- the latter I'd probably rule as a second opposed athletics check during a grapple instead of dealing damage, which puts the creature down after Con Mod consecutive successes.