Defense is by far better. At low stats it might not seem that clear but the way defense works is your defense stat is subtracted from their attack after a damage multiplier is applied. The easiest way to see this is use a -70% team with a very high defense. Everyone will start only doing one damage to you. If you can boost your maximum number of hits from 15 to 50, that extra bit of attack you are getting really is not going to matter as you will do more damage in the long run just because you are able to survive forever.

EDIT: Also, the ship battles are the easy part of the missions, the deck battles are where the real damage to your units occurs, and as you lose units, your buff later may drop significantly. I
have 378,184 whale ship attack without buffs or helper. Thus for every unit I lose, that is a lose of 3.8k attack later on. And that is a lot of attack if you are losing an entire deck squad (5 units = 19k attack, much higher that what a few rings if going to give you)