I suspect the Fiesta RS will be a better car than the Focus RS in most ways. Have to see what the spec is, but it's got a good chance of being a great car. Small dimmensions, but with a nice wide track (weight is nearer the centre) and plenty of rubber in each corner and less weight is all good news.

As for your question, the Focus RS is unlikely to plummet. I expect next year you will be able to pick up a high mileage 52 plate for £15k but the cars on more acceptable mileage will still fetch between £17k and £20k depending what plate they're on. The late 53 and 04 registered cars could still be fetching £20k next year if their mileage is good.