If you look at it purely from a physics perspective, I think it matters greatly. Imagine two strings: one that is rigid and does not move and the other which is as loose as a floppy shoe lace. When your finger touches the strings, as long as all things are equal regarding the force used to activate the string, the flexibility of your fingernail, point of contact, amount of nail used etc., the string which is rigid is easier as there is no movement. The loose string, by nature, will give way more than the rigid string. That is a gross exaggeration, but only to make a point. Now, if you alter your technique for each string type, things will change and you might find it easier with a less rigid string. But imagine the scenario in your mind and you will see my point. If someone with a more advanced grip on physics can explain if I am incorrect, please do as I'd love to hear it. In my mind, I think your nails and the amount of surface area applied to the string, etc. would be very, very important. A very rigid string with a very (VERY) flexible nail seems like an ideal scenario (but a nail that floppy would produce worse tone). Ugh, why is it all so complicated?