Get some long fins that allows you to replace the blades in the future. I would start with soft plastic blades (unless you're too heavy) and then, switch to carbon. A comfortable footpocket is very important. Try them before purchasing.
Btw some footpockets are easier to upgrade than others. My Beuchat foot pockets can be upgraded but, because Beuchat use shallower rails than others (e.g. Mares, Omer), it usually involves irreversible modification, such as cutting the support "tendons" and possibly glue. Beuchat offer their own carbon fins, so perhaps the blades for those might fit my Competition footpockets without modification? But hard to find and expensive, unfortunately Beuchat don't seem interested in the upgrade market.
My spearing fins (3 pairs + a spare blade, excessive) all have plastic fins and work well for my 100kg mass. If Beuchat came out with an inexpensive (less than £100), slot-in carbon or carbon & fibreglass upgrade set of spearfishing blades (with fitting kit) for the Competition footpockets, I'd likely buy a set to try. But it's not worth a lot of hassle and expense to me.
I too used to think replaceable blades make most sense. But often the fixed blades are much cheaper and can last decades. Plastic blades rarely break but it can happen. The price of a pair of replaceable blade fins plus a replacement blade or 2 and fixing kit too often exceeds the price of 2 pairs of fixed blade fins - and you might only ever need the one pair.
Buying a new pair of carbon fins - if you shop around - can often be cheaper and/or better value than upgrading an old pair. That way you can keep your old fins as spares too.