We JUST returned on Sunday from our first ever
Disney Cruise (7 nights on the Fantasy) followed by our first ever WDW trip (8 nights!), however, we have been to Disneyland, many, many times - in fact, we were married in California because I wanted to spend my wedding day at Disneyland

I think all of that gives me a fairly unique view. As DL vets, we were pretty overwhelmed with planning the WDW portion of the trip - all we heard and read about was how much bigger it was; how it was more, more, more; and how it was some how better. I feel like I need to defend DL and the idea of SIX WHOLE DAYS at DL
I did go to WDW with an open mind. I knew that it would be different and I can appreciate different. However, I actually ended up being underwhelmed by WDW. Yes, it is bigger, but it is so freaking spread out! You go on a ride and then you have to walk ages to get to the next one. At DL, you go on one ride, exit, and the next ride is right there! I found it fascinating that DL manages to fit into two parks what WDW fits into four, as there are actually a comparable number of rides and attractions between DL and WDW.
The California heat is different, the Florida heat exhausted us, we found we had to take way more and longer breaks, which was a pain in the butt because we had to get back to the resort :/ Taking a break at DL - simply stroll to your room, which is so much easier when wanting to go back. I found it weird that at WDW it took me longer from resort to park than my work commute at home
The crowds are different - we find way more season pass holders locals at DL, which makes for busy weekends but blessedly quiet weekdays...WDW seemed to be just steady busy. Also, we found a different attitude with guests at WDW as well.
Six days at DL - GO FOR IT!!! We are married with no kids, have been at least ten times and still get a 5-day pass

Out of the five days we end up taking afternoon swim breaks on two of the days. Six days would be great, you certainly aren't going to run out of things to do, and you also won't feel rushed or like you are missing out on something.
Halloween at DL is amazing, so much fun!! The nostalgia - I only really seemed to feel the nostalgia at WDW on Main Street, whereas DL seems to have Walt written all over it, so you are a fan of the nostalgia you will appreciate that at DL.
I enjoyed WDW and I certainly wouldn't turn down a trip there, but I can't see us going every year like DL - maybe once every five years? I came home from my WDW vacation needing a vacation, something that never happened with our DL vacations!!