I'll start by saying our sample size is as small as possible, but if our experience is helpful to you I am happy to share! We are about to embark on our first Regent cruise in June and are first timers navigating this included airfare option. When we booked 1.5 years ago we opted for the credit ($3,200 pp) because (1) we live in a major hub for numerous airlines so we knew we would have options, (2) we really wanted the flexibility of booking out to London and home when we wanted (we have friends in London we expected to spend a few days with), and (3) we knew there would be many direct flight options from our location.
With that background, opting out and booking out own was by far the best decision we made! The main advantage, we were able to upgrade our class of travel for a nominal amount. When I spoke with Regent about flights, they told me that on BA, our flight would be in Club World (business class). The confusion I had was that they were quoting us the $3,200 pp for business class when I was seeing First Class for $3,300. So long story short, we are going to be flying direct to Heathrow in first class rather than business class for only $100 more per person than the credit Regent gave us. This is a complete no brainer but I understand that our situation may be unique. Just thought I would share as some times it pays off to look around!