Take this with a grain of salt as I'm still planning my own first visit to DLP later this year, but --
If you have a chance to go to DLP and you expect to be able to go to WDW later, I would absolutely start with DLP. As BadPinkTink pointed out, it means you get the big "wow" factor for both parks without the risk that DLP will feel like a letdown after its bigger siblings. I'm also going to say that as an American who visited WDW many many times before ever going to Disneyland, I was told over and over that Disneyland is very small in comparison, that it would feel much more limited, etc. etc. And in the sense that there were only two parks, sure, that was true, but I didn't feel any less immersed in Disneyland than in WDW, and there was plenty there to keep us busy for a 4 night/5 day trip. I'm tempering my expectations again with DLP, because I know it's not the same as the American parks and I'm going to be happiest if I expect little from it, but considering plenty of people do continue to visit DLP there's got to be something that appeals to them in it.