I like the simple clean lines of the older model. I'm sure the paint shade difference is just a photo issue. There is something very attractive to me about JD lawn machines with the contrast of yellow and green.
In the photos, the decks sure seem different. On the X748, the deck is flat over the spindle and looks like it has a drive-over pad so you can disconnect easily and drive over the deck. The newer X738 has a bracket over the spindle that I think would be a challenge to drive over. However the trailing links from the front axle to the deck look more beefy on the 2nd model. The anti-scalp wheels and structure look about the same. Aren't the new decks made of lighter weight materials? To me, the deck is the heart and soul of a great mower.