I've worked on both islands.
I'd say the diving is better on Tioman.
Perhentians will be slightly cheaper for diving as they get more backpackers. Of the two island Besar and Kecil, the latter is the backpacker island. But be aware that theft is a concern on that islands. We went snorkelling in a secluded bay and the moment we hit the water somebody ran out of the jungle and nicked out bags. So be aware.
I'd recommend my old shop (although all the staff have completely changed)
Watercolours| Dive Center Malaysia they have a good system of diving. Try and visit Tokong Laut, the Sugar Wreck and Terumbu Tiga. Guides were not compulsory so make the most of that option.
Accomodation is slightly more expensive than Tioman about £12 basic if the exchange rates are still the same as when I was there last. They didn't use to have ATM's on the islands so you needed to take a fair wedge of cash with you. Any payments or withdrawals on Visa had a charge added.
Beer is expensive. 10RM (£2) for a 330ml Carlsberg
On Tioman if you want to dive stay on the west coast. I'd say ABC beach is slightly better than Salang, but that's because of the great pizza bar they have. I used to work for these guys
Scuba Diving in Malaysia on Tioman Island with B&J Diving Centre in Salang and in ABC - PADI, IANTD and DAN courses, Technical Diving they have the biggest boats on the island and got to more sites than anyone else. I'd recommend Labas, Tiger Reef, Golden Reef and Chebeh for dives.
Accomodation is ok, you can splash out and pay for air-con or spend £10 and get a double room with a fan. There are ATM's on the island. If you stay on ABC you can walk into the main town (nothing special aside from the duty free shop!) but on Salang there is nothing. B&J Dive centre used to allow money to be withdrawn on your card
Alcohol is cheap on the island as it is all tax free so a can of beer is about £1. But you can buy whole crates at the duty free in town. A good place to grab stuff if you will backpacking more in Malaysia.