I agree that if you have to buy one fin, medium is probably the safest thing to go for.
You might consider getting a really soft cheap one (you can get them for 80-100e) for technique training alone.
For speed alone the stiffness does not make as much a difference as you might think with an experienced swimmer. For example a guy who used to have a WR in 50m sprint that I know, for him the difference between a soft training mono and extremely stiff start mono was around 1 second. Of course in a world class sprint that is for ever, but for a freediver, I'd say the difference is negliable. Especially since the latter mono could be worn only for minutes and one lap before absolutely mincing the feet.
A freediver needs efficiency, not speed. A medium blade is probably the safest choice, although soft is much better for learning a good technique.
Of course then we have the problem of what makes a soft and what makes a medium. It is totally dependant on the manufacturer and sometimes even the individual blade specimen. I've seen "medium" blades that I'd grade as soft, as well as one's I'd grade as hard...
One more argument for going for medium is that soft blades are often very thin on the tip and can break quite easily. Especially in open water where you have a good chance of bumping into the bottom etc, a medium is much more rugged.
So as you see, it's really hard to choose. The important thing is that you get your first fin and get started with it. Just make sure it isn't too stiff, because a very stiff first fin can ruin your technique for years to come (been there done that
