I would recommend a trip to ko tao, because the dive center in samui would end up bringing you to ko tao. There is no place to dive in samui. Dive boat actually brings you to somewhere near ko tao. I've been to ko tao last month and samui last year.
Ko tao is more hip, in the sense, you got more freedom. You rent a bike, go anywhere u want, its a dive island. And more run down tho, compare to samui. ko tao is always constructing something all year round. but if the objective is to dive, then who cares.