Do French Stamps Proir To A940 Lose Their Value

Is it best to buy and collect MNH pre 1940 stamps or French WWII stamps?

What is best to buy is what gives you most satisfaction.

You keep coming up with these 'value' of stamps threads.

The better the quality of the stamp, the better your chance that you get something back on your stamps. Whether that is 120% of what you paid or 10% rather than 1% of what you paid is another subject.

As for losing value, that is anyone's guess. If an area falls out of favour, stamps will lose their value. What is a strong market now may be a weak one tomorrow.
Also, within any area, rare classic stamps may increase in value, when most stamps from the same time period or even the same series may lose their value.

If you collect stamps and cannot sleep worrying whether they keep their value, maybe it is not the hobby for you.

What NSK said.