26" vs 28" wheels for touring
I'd say that for loaded touring the place to start is tires, not wheel size. You need a bike that can accommodate larger tires. 32mm at a minimum and 40+ if you plan to travel on dirt roads or take on very heavy loads.

It's generally easier to find a 26" wheeled bike that can take larger tires, but if you find a 28" bike that can accommodate larger tires that will work fine as well. 

In theory if you build a 26" wheel and a 28" wheel with the same rim and spoke type/count, the 26" wheel will be stronger. But there are plenty of people touring with 28" wheels. 

The most important thing is to get a frame that fits you and has a comfortable geometry. If you're less than 170cm or so in height, 26" wheels might make more sense on a bike that small. 

Look at bikes that are built for touring and see what their specs are in terms of geometry and wheel size. Wheel size is the last thing you should be worrying about. Look at the specs for this bike 

http://surlybikes.com/bikes/long_haul_trucker

In the smaller sizes it comes with 26" wheels and 28" wheels in the larger sizes.