Figuring out the brake difference is easy. For example on a 597 to 590 conversion there is a 7mm difference in diameter therefore a 3.5mm difference in radius so the brake pads will have to drop somewhere around that amount depending in rim design. Not really too much for most brakes.
The overall diameter of the wheels plus tires is about the same for the two sizes, the 590 tire being about an 1/8" (6.8mm) of an inch taller, sometimes a little more due to heavier tread.
Therefore fender lines are going to be about the same any problems that will arise are likely to be in clearance for the additional width. This in most cases on bikes that use the 26" x 1 1/4" size it is not a problem as during the time period of use bikes had generally fairly liberal clearances and the width difference is not huge.
Now 650B is 584 mm diameter and the brake pads will have to drop that much further, which may mean changing brakes in some cases. Overall diameter and clearances will depend on the tire size chosen as there are quite a few choices in this wheel size.
So for ease of swap the 26 x 1 3/8 (650A) gets the nod. I only know of one alloy rim currently produced in this size, the Sun CR18. Utility grade tires are readily available almost everywhere and some lighter tires if one does some searching, the size is still quite popular in a few countries in Europe and also Japan.
For variety of aluminum rims 650B has had a bit of a resurgence and has the advantage. There is also a good number of offerings in tires in various weights and widths.
If it was me I'd go the 26 x 1 3/8 route, it's a proper English size and likely an easier swap and the CR18 rims don't look too far out of place.
Marcus Coles
London, Ontario, Canada.