Thanks for the detailed write-up Ngaire. We would like to try the new Seabourn ships one day - we sailed on the Seabourn Spirit in '98 to Indonesia and rank it as one of our favorite and most interesting trips of all time, but now prefer the updated cabins and amenities of their newer ships.
The spa charge at first sounds odd, but if you have a treatment then you get use of the spa before and after, so that is probably OK for most. Not sure I'd pop for an extra $300 to have access to the sauna/steam etc.
For us, itinerary would rule the decision and we always enjoy trying new experiences.