Thanks for your input!
I'm aware that additional discounts might be available -- especially for those who book late. In fact, I often take advantage of such discounts since they often let me cruise twice per year instead of only once.
Although I enjoy cruising, I'm actually asking the question as an investor who holds an interest in both companies, wondering whether there's something significantly superior to the pricier product. If so, Princess will generate exceptional profits. If not, Princess will be holding a "fire sale" on unsold berths as savvy passengers flock to the less costly competition. It could also be a situation in which Princess has more built-up demand from a more established presence in the European cruising region, in which case both fares might be appropriate but the fares will equalize one way or the other in the future as Celebrity's presence becomes more established.