Both are high quality leathers and here are the characteristics of each as I understand them:
Laguna - has an aged "pull-up" effect to the leather which gives it a "vintage" look. It is resilient and long lasting
Nappa - softer and smoother than most leathers and is processed so that the natural leather grain is preserve. It is treated with water-soluble colorants which makes it fade resistant, so excellent for automotive seating. It is however, susceptible to scratching
I have the Alcantara/Nappa combo in my Scat - it is very nice. At first, I was unsure if I would like it, but I have now come to prefer it to the full leather seats I have had in other vehicles because with spirited driving, you don't slide around on the Alcantara.