Also, although you're saying wheel weight matters, you're also looking at substantially heavier tires compared to your 25's. Just based on a rough estimate by circumference without looking anything up, 2x heavier.

Edit... more like 1.6x heavier. Soma Grand Rando superlights are 300g, most others in 650b x 42 are 350-400g. Compare to 215g ea for 700c x 25mm GP 4000S II. Bigger tubes, too, 149 grams standard and 108 grams light. Compare to 65/112 for skinny 700's. So you're looking at roughly 3/4 lb more rubber, for 2 lb total.