No need to toss a coin a one pipe will work just as well as a two pipe system.
However what you should seriously consider is the age of the pipework, you can bet you bottom dollar it's a lot older than 17years, maybe 30-40years even, you could get a clue if the pipes are imperial or metric.
Visually check what condition it's in and if any is suspect change the lot, in which case go for the two pipe system.
