I'm moving mine horizontally too. I was looking at my 55 long today and I thought maybe it might be cooler in the right corner, because my heater is in the left corner.
i had my heater on the opposite side of the tank as my filter intake in the vertical position. the heater would get bubbles on it when it heated up, this bothered me. then I moved the heater right next to the filter intake vertically, the increased waterflow past the heater prevents bubbles from forming on the heater at all when it heats up, I have a feeling no bubbles on the heater allows for better heat transfer into the water. not to mention the heated water is immediately circulated throughout the tank by the filter resulting in a more stable temperature. also the heated water going directly up the intake might even help the bacteria colony in the filter since I believe they like warmer temperatures.