C4 30 or C4 40

Hi everyone!

I have a question, someone might shed some light and help me.
I'm planing to buy c4 Falcon and now I'm not sure which one should I buy.
Regarding stiffness of the blades - 30 or 40.

Now, I have been spearfishing for years and in last 15 years I had one pair of Sporasub H. Dessault fins - I hope someone still remembers those: black footpocket, red blades (plastic).
What I have done to those fins is for another story but - they have survived to this day.
I have purchased Sporasub Pure recently on eBay and that was a mistake.
I got them used, blades are of different thickness (stiffness), one (thicker) is showing a beginning of crack at back (middle), it was poorly assembled, edges are falling off (I guess some glue might do the job)... in short - disaster.
So, I'm going carbon for sure, unless there is better material for spearfishing.

Depths - 20-30 meters, mostly waiting technique... I'm 5' 11" (180 cm) tall, 83 kg (182 lbs)...

any comments or advices regarding difference between c4 Falcon 30 vs. 40 ?

My first instinct was - "buy stiffer" but I'm not sure. My old H. Dessault fins are at least 40% softer that I would like, but I know how bad can wrong choice in this area be.

Thanks!

In addition, this is the photo of one of my Sporasube Pure Carbon. You can see the cracks that started at the back and on the sides - poor job of assembling the blades and foot pockets.