I am copying a very thorough answer from Prof. Dheeraj Sanghi's (IIT Kanpur, CSE) blog here: http://dsanghi.blogspot.hk/2011/...

Should I prefer a Dual-degree program or a BTech program.

If you are comparing Dual and BTech in the same department, say Dual  (CSE) versus BTech (CSE), then the answer is very simple. Go for BTech. I  think the dual-degree programs are fantastic, but why go for an early  commitment. And if the MTech part of the dual degree is in a specialized  area, that should be avoided. When students don't even know the  difference between Computer Science and Electrical Engineering, asking  them to commit that they will work on "Power" or "Wireless" or "VLSI"  after three or four years is simply ridiculous.

Now coming to the more interesting question. I prefer CSE over EE. But I  am not likely to get admission in BTech (CSE). Should I prefer Dual  (CSE) or BTech (EE). You could replace EE and CSE with any two departments, and the answer  will be same. It depends on how strongly you prefer one department over  the other (CSE over EE in this example). And to test how strongly you  prefer one department over the other, I will ask a theoretical question.  Be honest in answering it. Your BTech (EE) program will finish in May 2015. Suppose I were to offer  you admission in BTech (CSE) on the condition that your program will be delayed by three months, and you will actually graduate in August 2015.  Would you then prefer BTech (CSE) or would you still prefer BTech (EE). Basically, if you prefer EE in this hypothetical scenario, then it is  clear that your preference for CSE is very minor, and you are not  willing to pay even a small cost of three extra months to undertake a  CSE program. Then you should prefer BTech (EE) over Dual (CSE).

But if you preferred CSE in this hypothetical situation, then we can  continue this game a bit further. Now, I further suggest that you invest  9 more months in the discipline for which you have indicated a  reasonably strong preference. In these nine months, we will most  probably give you more than one lakh rupees in cash, we will give you a  degree that normally takes two years to complete, and a degree that  opens a lot more doors for you in industry. At least in CSE, several  multinationals have setup advanced R&D labs in India, and they hire  MTechs and PhDs only, giving a very good work environment and a  lucrative career. I am sure there are such labs in other disciplines as  well.

Hey,

Dual degree is a 5 year programme. It involves a B.Tech. degree and an M.Tech. degree of your discipline. In many IITs, this is a matter which is decided in the beginning during Jee counselling. In some IITs like IITK, all the seats have been converted into 4 year programme seats and one can choose whether or not to go for a dual degree towards the end of 3rd year, which changes the course into 5 year B.Tech-M.Tech programme.

 
The choice is personal and based on interest in exploring more of a discipline in a greater detail. That's the advantage of dual degree. Whether or not this gives the students an edge over B.Tech. students during placements is something I've never heard of.
The choice of 5 year BT-MT programme is never based on getting better placements but on exploring the subjects into greater detail.