personally, depends u want the crowd or u just want to be @ peace. 
lang tengah smaller not so crowded. bring ur chick then good just to chill out.
redang bigger more ppl, better facilities. join the big crowd go snorkelling and end up marine park crowded with legs and hands
beach wise redang better wider and the sand is finer generally, sealife watching wise go lang tengah.

i wonder is that coral poisonous?