For the most part I mine Ethereum on Nanopool, but when I made the move back to mining ETH over a week ago, I put half of my rigs on Nanopool and the other half on Ethermine and let them mine on both pools for one week. I've done this in the past and the differences between the two pools wasn't that big of a deal. This time however, there was a noticeable difference.
My accepted share times on the east US Ethermine server were better at ~77 ms compared to the east US Nanopool server at ~154 ms, but that wasn't anything out of the ordinary. My current effective/calculated and average hashrates on both pools matched or was close to my reported hashrate which also wasn't anything out of the ordinary. The difference between both pools was the amount of ETH mined. The rigs on Ethermine consistently mined 10-15% more ETH than the rigs on Nanopool every 24 hours.
Considering that the hardware, software and hash power I put on Ethermine and Nanopool was identical, I was pretty surprised with the difference of ETH mined between them. I haven't gotten results like this before and I'm not sure how to explain it or how long it will last, but right now I have every rig in my farm mining Ethereum on Ethermine.