Subsonic said.. One's a 36, the other appears to be a 28.
Too easy
OK I'll put more info down.
You can write a book on fins and how important they are to the performance of a board.
I'll give a snippet of what to look for.
The 36cm fin has some good qualities compared to the 28cm fin:
Earlier planning due to longer fin and more surface area. Therefore it will push the board onto the plan quicker.
You will also notice the 36cm fin carries the more vertical plan-shape to the tip giving even more lift.
I also like G10 much more over a moulded fin. The 36cm is G10, the 28cm appears to be a moulded fin.
Moulded fins damage way easier than G10 and G10 wears much better as well if your fin drags on the sand.
Also if you know what you are doing you can also reshape G10 fins, not so moulded fins.
Now to speed and control:
The smaller fin will have way better control in higher winds due to less depth and area and plan shape of the fin tip.
Control = better speed.
More area than is needed = drag. Drag decreases speed.
You haven't shown a view from the front of the fin which would show the thickness of the fins and where the thickness is placed in the vertical sense.
A thick foil lifts the board onto the plan quicker than a fine foil.
A fine foil has the capacity to be faster if done well.
I'd buy a few fins for the board and learn for yourself.
You will also enjoy the better wind range that you get from having the choice.
