They are both breathtaking, but on my visit a couple of years ago, I was much more impressed by Moraine Lake. However, you can likely visit both at the same time, as the road to Moraine Lake is right near the parking lot for Lake Louise. On my visit in late August, the entrance to the road to Moraine lake was blocked and they would only allow cars to go in as other cars exited, as the parking area at Moraine Lake was full. If you plan to go, go early in the day.