The 1.5ohm carto does seem to stay slightly cooler longer, but not a whole lot. I just did 15 consecutive large chain vapes right now on the 1.5 before it got really hot to the touch. I don't rememberbeing able to go that many with the 1.25. So you might get a couple more vapes before cooldown. One tip I found with the 1.25 was to loosen the carto just a bit, and it will allow more airflow. More airflow = stays cooler longer. Of course that means less of a ratio of vapor to air, but it did help keep it cooler.
I did a brief side-by-side of both the 1.5 and 1.25 when I transitioned, and the biggest noticeable difference for me was the excessive popping and splattering, especially as the carto ages. I have not had the 1.5's splatter at all, period. Also, since they have more resistance, it will theoretically give me a longer battery life as well. I interchange between 4 Kgo batteries all day, so I am not able to determine the exact minutes it will give you, but that is another advantage.