Hey all, I'm back.
Anyway...since it was my very first dive with one shop, I can't really compare which one is supposedly better. Here's my observation. For all the veteran divers here that knows even more than I, correct me if I'm wrong
I went to Gill Divers, personally I felt the instructors do a good job on explaining everything during the pool practice and the open water dive themselves, as well as their own skills in the water, but I also felt that it was inadequate.
If I have to be really objective on WHY I think it was inadequate, was because my group was kinda like very difficult to learn the techniques...the instructor spent a LOT of time trying to get the other 4 divers of my 6 newbie diver group to get all the skills right and getting them to learn their buoyancy correctly. (as Stevie mentioned, they're all quite safety conscious)
Apparently all the PADI shops (maybe not all) always organizes these sort of regular trips together. Therefore during my trip to Tioman yesterday, only around 15 people are from Gill Divers, and maybe 25 people are from different shops. (e.g. there was also from Deep Blue Scuba, I heard) They're all doing the Open Water Course.
In Tioman itself, we went to Salang Beach, where most of our dives are done in Monkey Bay, as that's the most beginner friendly area, I'd say.
Stayed at Salang Beach Resort, with minimum amenities (bed, running water, no heated water, aircon) and also "rationed" food for breakfast, lunch and dinner with freely available fresh water to drink. But as our instructor told us, safety as well as the cheapest price for all our lodgings goes beyond comfort for such a big group, so we got by with the cold showers and rickety floors of the chalets...
I had a great time diving on the second day, mostly because I've pretty much controlled all the basics, and we're able to finally do some actual diving. And of course, the other 4 people can finally really control their dives albeit still not as good.
Next, I'm going to do the AOW so that I can go even further. I'd like to go to Redang, but apparently even around Salang Beach, there are still plenty of places to see. One AOW course diver told us that he went for a cave dive, as well as a night dive. So of course there are different experiences for diving...even if it's the same place.
Anyway as for which shop you should go for, personally, there's no difference which one. They'd most likely go together in the end...and it's most likely depend on luck whether you get an adequate instructor, as well as a good group. I got a pretty good instructor...I didn't get a good group though

wasted a lot of time just kneeling on the sea floor waiting for the others to finish their skills and such...
Hope that helps!