give the aptitude test wiithout a second thought... because theres no harm in giving it and it hardly needs any preparation.. 
Then you can decide whether to go for architecture or not.. but if u miss the test then there wont be any 2nd chance.
In the 5 yr course ull be taught the best and the optimum way of designing living spaces taking into consideration all the funtional and aesthetic considerations. ull also be given a thorough knowledge of structures and construction technology.
the main diff between archi in normal collges and archi in iit is that in iit the course focuses largely on technical based architecture rather than the aesthetic based course elsewhere.
future prospects are good too... if u can seriously be a good architect having his own firm ull be earning money an engineer can only dream of... 
that aside there are good placements in consultancy and finance firms frm iit and also core architecture firms..
and do give the aptitude test.
Hope the answer helped.