I love my Debut III, but probably the hardest thing to love about it is the tonearm, which is very lightweight insubstantial thing, and I'm sure would be the main limiting factor on cartridge upgrade options - I'd have to upgrade the counterweight in order to fit a 7.2g 2M series for example, and better just forget low compliance cartridges. I guess the 'Carbon' upgrade goes some way to curing this, and if it has a 2M Red fitted provides an easy upgrade path to the Blue.
However as AI says, having the 2M Blue already fitted on the Fluance is a serious consideration in terms of value, provided all other components are more or less up to Project / Rega standards.
On that subject, I always find it strange with WHF deck reviews in particular that the cartridge is sometimes given very little prominence, sometimes barely mentioned, when it is surely of crucial importance in SQ, literally at the business end of the deck. I know a decent arm, stable power supply, platter etc. all contribute, but sometimes I think their preference for Rega over Project is more of a Rega vs Ortofon cartridge preference.