As stimix said, redang more asian, perhentian more ang moh.

Perhentian you can walk in choose your accommodation if no peak period, food sendiri cari as many restaurants along the beach. Redang mainly all sold in packages include food, ferry, accommodation, snorkeling trips by the hotel.

Redang resorts are better quality. Perhentian resorts more basic style.

Redang long beach has quite some happening stuffs at night, but Perhentian long beach is more happening. Perhentian has a few beaches, so you can choose which beach to stay. Redang has one main long beach and a few hotels on more secluded beaches.

Anyways, both Redang and Perhentian is very very different compared to Langkawi.

You want relaxing, go to Perhentian Besar island or go to one of those resorts with more secluded/private beach on Redang.