Two steps or One?

I have to climb up 4 to the forth floor about 4 times a day, and I was wondering, what is better for me, physically? Should I be taking steps one at a time, or two at a time?

1 at a time better aerobically.2 at a time a better muscle mass builder.

What do you want from the “workout”?

But, it seems to me, is not a good idea if you have knee problems.