I have been using p/s cameras all the way till now and always wanted to get a DSLR.
Why?
Now that i have made my mind and budget,
Never mind why...
I am reading all the stuff on internet about various camera bodies.
I doubt you have read ALL of it by now, because there is soooooo much of it.
I am stuck at choosign the right camera body which would prove useful on a longer run.
The most basic, entry level offering from any of the major manufacturers is going to be more than enough for the 'long run' - more than enough.
I'll say it again, because new buyers often have community induced blinders on... any of the very basic intro, beginner, noob, 'point and shooter crossover' cams will have far more potential than you will be able to coax out of it for the next 5-10 years.
I have short listed T3i and D5100. Which body should I choose?
That depends - which camera SYSTEM do you want to be in? The actual bits and pieces, including camera body, are so unimportant now that you won't go wrong. (you should have read by now, since you have read 'it all' that you need to hold the cameras in question in your hand for a while - see how it feels since you will be holding it a lot)
I definitely need the swivel LCD hence these.
Nice, one of mine has a swivel screen, great on the tripod.
Also I would be using my camera for Landscape and friends and family photos. I intend to take my photography seriously and would collect some useful lenses as I learn more.
Warning - getting a dslr will require some basic learning to fully utilize and enjoy all the capabilities - and shortcomings. This is not a bad thing, to me it is a wonderful thing - but many P&S up-graders don't realize just how much those little cameras were doing for them when it comes to a 'pleasing' outcome.
Just wondering which would be right camera on longer run.
Neither. The camera, and it's accessories, are not the weak link right now - you are. Your biggest gains in 'quality' will come from your basic understanding of photography. It's not rocket science, far from it, but there are a bunch of areas you will want to study and understand to take full advantage of the new camera.
Is it true that T3i has a motor in its body, while D5100 does not which would make the no. of lenses available for T3i more and cheap as opposed to D5100. Please pardon my ignorance here.
Yes, and no, respectively. Yes the Canon has an auto focus motor in the body, the Nikon does not. No, there are more Nikon lenses available - especially in that motor-less mount the Nikon uses which will mount more types of lenses than their flagship D3/4, more than the Canon's, etc... Of course, many of these lenses are already manual focus lenses, or would have to be used that way on the Nikon. Also metering is another thing to consider.
There is even more to it, but like I said... there is a lot to learn as far as basics. It's an enjoyable craft and art.
Study the systems more - a lot more. Hold them both in your hands - for more than 60-120 seconds when you are at the store. Also, understand that a solid understanding of the basics, strengths, and limitations of the gear in question will far outweigh any advantages the equipment itself will bestow upon you - by whole factors and leaps-and-bounds.
Really - no kidding... Your understanding and implementation of the basics will do FAR more for you than which particular system you choose. My advice? Get one on the cheaper side of your budget, get a few good books or courses (online or real time), get a flash, and a nice bag for the equipment to live in. Good luck!
and good ones!