Congratulations - stairs are awesome exercise!
Do either of you have a heart rate monitor so you could check? Otherwise, you could both get to the top, stop, and check the pulse in your necks or wrists for 10 seconds. Multiply by 6 and you have beats per minute.
Then to be really
scientific, switch and each of you do it the other way, take your HR, then compare which way made each of your HRs higher. You have to compare yourself to yourself because your hearts might be in different condition and one may be naturally faster or work harder doing less work.
Honestly, I think both ways are probably equally tough but if you check your HR doing it both ways, you'll know for sure.