Excellent question, reader! And one I researched myself before purchasing my Polar F7.
The only difference between the F6 and the F7 is that the F7 chest strap is a much more comfortable material than the F6 plastic strap.
Comfort is extremely important to be while working out, as I tend to get irritated easily by things that touch me. For me, it was worth it to spend the extra $20 and get the F7. Again, the material of the strap is the only difference. You even get an F6 handbook with your F7!