I assume you've read the notes on laying blocks over an existing concrete base, Roger. bearing in mind the comments about cutting out for thresholds, how much of the ex conc would remain?
This is one of those jobs that's best decided on a case-by-case basis. Flexible construction is by far the easiest method for DIY'ers, as rigid can be messy and time-consuming, and you have to use clay pavers. So, unless there's good reason, I'd go for flexible.
How feasible would it be to jack-hammer out all the ex conc?