Charlie, other than the probes, no reason at all! I think the 1503CPH was one of the best units Auber made. That's what I've used the most, and never had any issues with it, at all! The D-models use the GPH technology, but I'd wait until your 1503 has a problem. There are a few things the GPH units have, that are nice, like probe calibration offset, 8 programmed recipes, and the ability to just enter a single temp and set it to "on." Other than that, they work the same. The 1503 is a simple, solid unit.