The major difference will be in the longevity of sails made out of the different materials. It is very simple. Dacron will not hold its shape as long as the kevlar. The dacron will look ok for perhaps 10 or 12 years, despite being blown out of shape after two. The kevlar will start to delaminate and disintegrate after three years. You may be lucky if you can still hoist the kevlar sail after 5 years without it blowing apart. Since you're paying, you get to pick the sail you want.
In such a small boat, the performance difference between two NEW sails of different materials will be minimal. If both materials are cut to the same design, they will perform similar to each other, as long as they hold their shape(s). Sails on a 8m boat are simply not big enough for the materials to make a big weight difference. If the mast was 100' tall, THEN, there'd be a weight difference, which might have an effect on performance. Hope this is helpful information to you.