hello there!

First of all, if you are in the 12th standard, then you cannot do a direct Masters in Physics. Either you have to enroll in a BSc /BS c (http://b.sc/B.S) ourse , complete it and then go over to MSc or you can get into some integrated MSc programs (like the ones offered in IITs ). Considering you can get into any of these programs (BTech/BSc/MSc), the question is what you want to pursue. If you want to do a BTech in Electronics you can work in different areas like semiconductor devices, networking and communications, VLSI and embedded systems, robotics etc. If you want to work in areas in which Physics grads do, you will have to get trained in those topics ,by yourself, since you wont get much exposure in those topics during your BTech. Many colleges like IITs offer the opportunity to do minors in another stream by taking up electives... so if you can do that ,its pretty fine. After that you can go onto Masters in Physics and you will be working in the same areas that physics grads do.I know some guys who did that, since they enjoyed Physics more than core electronics.Let me warn you, doing that is not easy.On the other hand , if you take up physics, the same case will apply, of course in the opposite way. Being an electronics major myself, I cant comment on the full opportunities in physics, but I am sure that you can work in core electronics, after studying physics, if you can develop the necessary skills.Physics is so broad that you can work in any area in future. I am not entirely sure about working in Mechanical/ other specialized engineering disciplines after studying Physics,though.

The main factor is obviously what you want to do in future. Its okay to be confused at this stage. But ultimately you will have to make a decision.The thing is that ,you can basically work in any area in future, provided you have the necessary expertise...it really doesn't matter in what area you do your Bachelors.If you are bent on getting a job and a fat pay-check right after your undergraduate level, I would say BTech gives you better opportunities. There are pros and cons in everything. So just follow your passion and do whatever seems interesting, no matter what others tell you to do.

Best of luck.