I am going to go against the majority and say DEFINITELY Disneyland for your first visit- especially with a 5 year old. You won't get the total immersion experience that comes with staying at a WDW resort but he will be so bowled over by everything he really isn't going to care where he is staying, as long as it has a great pool! (Check out Hojos for a great water playground that our boys love)
We first went to DL with a 6 day pass which was fine. then last year we got a 14 day pass, thinking we would never use it all, but we still managed go to the parks on12 of those 14 days! and that was before Carsland opened! with a 5 year old you will still be wanting to take a break most days and head back to your hotel for a rest in the afternoon. this is much more easily done at DL thanWDW where the distances are so much greater.
Save yourself the extra flight and expense and start with DL- it's Walt's original, after all! save WDW til he is older and can cope with the bigger distances and longer days.
I guess starting with DL, it will be a load cheaper and quicker to get to the theme park, I guess it opens up a second trip later on to WDW quicker. So more Theme parks I guess. Will check out some of the hotels at DL