I don't think there's much in it. Perhaps it is easier to get any old 26" tyre rather than a 700, but then again if you're not going to a really exotic or remote place, I'm not sure it's a real factor. Perhaps 26" wheels are theoretically stronger, but I think component quality and build quality far outweigh this generalization -suffice to say, given good quality wheels in both sizes, both should be strong enough.
The only thing I believe is a factor is whether or not your size impacts the design of the frame and hence the wheel size chosen and the comfort and fit of the frame. Personally, I hate toe overlap and since I'm not very tall, with some of the 700c frame designs I'd get toe overlap -most especially once fenders are fitted. Some people don't seem to mind toe overlap -I'd much rather have a frame design without it.