ID as the name implies means that they are going to do more than renvoation work. they are supposed to come out with more innovative designs...no doubt they claim that designs and computer modeling are free but somehow they manage to add on every unit of service charges.
as you see, renovations has many phrase that need to be carried out. from laying of tiles, pipes, carpentry work, painting etc...ID will help to co-ordinator each phase of work to be done in proper sequence. that's why ..we are paying for these too.
with the above extra service provided by ID, hence it will be more expensive than contractor.
but the benefit of contractor, is you get to speak to the person doing the job directly. and you are paying money to them directly. as long as you talk to them nicely, once in a while buy them coffee, cigrettes - they will be very happy to assist you and provide you with lots of good ideals.
sometimes, no matter how good IDs can be, they lack of experience which contractors can easily has more than 20 years of experience. most of them start as workers when they are young and start their own business or take over the business from their ex-boss.
the bad thing about contractor is, you need to come out with your own designs. if you are thinnking of some new age designs, don't expect contractors to understand what you mean. in their mind , their concern is practical.