Jez
Sorry for the late reply-been working away this week. I reckon you're better going with the #5 (or 5.5!!) You can get the same degree of finish as the smoother (after all,the bench planes are all built to the same high standard).
As you say, get a second iron and you can use you plane as "two" different planes by altering the mouth setting/iron.
Good luck with your purchase, you won't regret it.
Philly