I can't speak for India, but in the states finding 26" tires is slightly easier than finding 700c in towns without bike shops. However, I don't think it is enough of a difference to matter, even though I was stranded for a day in Idaho because I needed a new 700c tire. Finding appropriate tires for road touring in either size (slick 26" or wide 700c) can be a challenge regardless.
Instead, I would pick the wheel based on its other properties. On smaller bikes, 700c wheels can over some amount of toeclip overlap. This should never be an issue when you are riding above 8mph, but slow speed turns can allow your foot to hit your front wheel. I make a lot of u-turns and other "parking lot" manuevers on tour where toeclip overlap becomes an issue, but simply remembering not to put my outside foot forward solves it. 26" wheels can be built stronger because the wheels are smaller. If you plan to do much off road riding, 26" is the way to go.
I have a Trek 520 and I have had no problems with it, at least since I changed the saddle and rack.