Hi! My fiance and I are also sourcing for ID to do up our future place. As we still have lots of time, we did some homework and here comes our advice (from whatever little we knew):
1) We think ID firm would be better. They could give us rather practical advice on the space planning and little tips on where to place the furniture etc. One time, we talked to an interior designer about the layout which we thought would be quite nice, they actually pointed out to us that we should not place a fridge in front of a window, because the fridge is too high and would look ugly. Another thing he pointed out was that if I put my tv console under the window, the glare may cause discomfort to our eyes when we watch tv in the day time. These are just some examples of details which we have missed out, and ID would be able to advise us on. We did not even consider engaging contractors, because we heard they only do what we asked them to do, and we heard sometimes they even may have problems following instructions.
2) We are more interested in modern oriental style (some IDs called it "modern China"). And I heard that themes like modern oriental, resort, Balinese and etc. are more expensive to do than the simpler ones like minimalist (some IDs called it "modern comtemporary"). I heard minimalist needed only 25-30K to do the whole unit, so I suppose you would expect to pay more than this for Balinese. You may go to any of the establised interior firms, which could be easily found in the classified. The most important thing is to talk to the designers to find out if their design concept suits you, and also whether the pricing is within your budget. One of the designer we talked to seemed very well-verse in these Asian styles, his number is 97403624 (Can't remember his name, saved as ID-Asian in my contact list).
3. Firstly, make sure there is no hidden cost. Many interior firms quoted very cheap, but we heard there are lots of hidden cost. Secondly, check the quality of the material used. Some of the ID form we talked to claimed that they charge more because they use good quality material. If that is really true, we wouldn't mind to spend some extra. The problem is that we are not very familiar with the classification of materials, so perhaps it helps to do some googling. Thirdly, ask for discount or some free stuffs. Most of them COULD give at least some discount. One ID offered us free false ceiling, not sure how useful that would be, but who would mind free stuff?
All the best in your attempt. Please let us know if you come across any good ID firms, we are still on a lookout, eventhough feeling a little exhausted after talking to a number of IDs.