Greetings Carlos and welcome to SB! Your adventure is just starting and it is awesome indeed. The fin question has many possible answers as you all ready know!
Proper sizing, proper fit, and style of diving that you intend to do factor into your decision. Best advice I ever got personally was to try as many different ones possible before you have to buy. I realize sometimes this is not possible but if it is it can prevent buyers remorse. Needless to say I have had it a few times.....
No worries my friend e-bay, craigs list, SB, have plenty buyers or sellers of fins!
I like to buy from LDS and have a incredible working relationship with mine, I try to remain very loyal but have purchased from SB some gear and it was great. Just as it would have it, it was a set of fins for my wife. It was a good deal and I saved $$$$!
Fins were in amazing condition, almost new for less than half price!
I would not hesitate to take such a deal if it was what I wanted!
Your instructor can be a valuable resource in selecting the right fin for your local conditions surf, current, etc. Take advantage from his experience as well as local dive club members who have been diving in you local scene. Then just go for it!
I know I have welcomed you to SB but there is a larger community that you are joining, the SCUBA DIVERS man is it great! Get ready for awesome friends, good times, and unbelievable dives! It is just around the corner!
Safe training and learn / practice your skills often they will serve you well!
CamG Keep diving....keep training....keep learning!