It all begins with the kind of riding you want to do. Over the years I have found that I increasingly prefer small roads and sometimes stretches of gravel roads etc. The reason for this is quite simple: traffic has become so much denser that all but the smallest roads have become dangerous territory. When I started touring in the later seventies I often would ride my Viscount Aerospace Pro (yes that one -still have it) on 32-35 mm tyres along pretty major roads with smooth tarmac and little traffic. I don't need to tell anyone you cannot do that anymore. For me, speed of transportation is now quite irrelevant, but involvement in the scenery is what matters instead.
So I now ride a different bike: it is still a steel drop bar bike, but it is now a custom frame with oversize tubing, 26 inch wheels and a Rohloff hub. I prefer 26 inch wheels for this kind of riding, as for the same weight they allow wider tyres offering more comfort, and better grip on bad road surfaces. 26 inch wheels may indeed be marginally slower (but only marginally so), but speed really depends much more on tyre quality or when you are climbing on luggage weight. So yes, narrower 28 inch wheels may be a bit faster, but they are not equally adept at bad roads. The world has changed, and so we need different bikes.
This is not to say there is no room for narrow tyres and 28 inch wheels. A lightly loaded Audax bike can be fun indeed, but only for hostelling or really ultralight camping and for riding on preferably decent tarmac. In fact, I am now putting together some ultralight camping gear precisely for this, first with the old Viscount, and if I like it I may get a new audax bike for it.
For alround touring use I do indeed believe that the 26 inch wheel is the most versatile. Fit it, as I do, with 26x1.75 Panaracer Pasela's and you have a very fast bike that can handle moderate loads. For heavier use there are the 50 mm versions of the Schwalbe Big Apple, the ordinary Marathon, the Schwalbe Supreme and Extreme, and for ultimate third world conditions, the Marathon XR. For semi off road there is the Schwalbe Hurricane. In short, you can transform your bike according to the conditions. To some extent you can do the same with 28 inch tourers like the Dawes Galaxy, and particularly if you fit wider tyres they will still be quite ok on gravel, but not to the same extent as wide 26 inch tyres.
Willem