At pressures you can achieve with a tubeless setup the 28s are the way to go. But rationalization,appeal to authority, and cognitive dissonance rule the cycling world so let me give you some actual data to guide your decision. 
The weight difference between the 25s and the 28s is going to be on the order of 30 grams total. I consider that de minimus. On a climb you’re not going to know that difference exists. Especially considering the next point.
At tubeless pressures the 28s will have a couple watt advantage over the 25s in terms of rolling resistance. Maybe more than that if the road is a little rough. During a climb rolling resistance is relatively more important because you’re at a slower speed and aerodynamics matter less (as you know, just mentioning it for the sake of being complete).
And, finally, 28’s have the intangible advantage of added comfort. You should ride them back-to-back. 25’s first then 28’s. If the rides are proximal you’re going to notice that the 28’s MAKE A HA-YOOG difference.