Heat Rises and preheats your next bead
I'm no welder by any stretch, but common sense tells me that your up hill method is better/has more pen. than a down hill weld cause the heat is traveling up and preheating ahead of you better than it would on down hill travel.
You could prove that out by quickly appling a torch to the middle of a vertical plate and measure the temp. diff. top & bottom.
My guess is the bottom will be cooler.
I enjoy your vid's and hope you make more of them.