As previously mentioned, with only a $400 price difference between your Signature Plus AP and the Premier Passport, doing the upgrade for a week long trip is a no brainer. You'd spend much more than $400 for a 7day ticket that matches entitlements with the Premiere Passport.
As you correctly surmise, even if you don't truly understand it yet, the two places are very different. I grew up at Disneyland - I was literally there on opening day. I wasn't 2 yet and remember nothing, but I was there. I love Disneyland. I enjoy WDW more for entirely different reasons. Living in Portland, we actually go to WDW more than Disneyland.
Don't be afraid of all the hype you'll see here and on other boards about how much you need to plan. Is planning important? Sure, but your trip will not be a complete disaster if you're not able to make dining reservations exactly 6 months ahead of time, or if you can't get every fastpass you want for the exact 5 minute window your master planning spreadsheet says you must have. You can (we do all the time) have a wonderful trip without planning much of anything more than a day or so ahead of time. Use the tools available (
MDE, FP+, ADRs) to help you lock in the few things that are really important to you or that are time sensitive. Don't get overwhelmed feeling as if you need to plan and schedule every waking moment of your trip. That laid back vibe you mentioned is possible at WDW. It's a mindset. Nothing more
We have a 10 day trip planned starting on Oct 27th. That's 62 days away. We have exactly 2 ADRs booked. Both are for brunch on each of the 2 Sundays we're there. I'm not planning on making any others until we're on site, and then, only for the next day - maybe. There is always somewhere to eat. Likewise, FPs aren't really that big a deal. It's great to have them, for sure, but unlike a whole lot of my cohorts here, I'm not adverse to waiting 30 minutes in the standby line to ride something if i really want to. My wife and I have a great time taking to people in line and *gasp* to each other. After 30 years of WDW trips we tend to do a lot of reminiscing about doing such and such with the kids, or "remember when 20,000 Leagues was where the Mine Train is now"?
All that to say, enjoy the journey as well as the actual stay. Feel free to ask questions. Read the stickies about FP+, tickets, etc. Lots of great info there.
And try to keep in mind that the size and scope of WDW is humongous compared to Disneyland. Tidbit to demonstrate that point: think of everything inside the berm at Disneyland. ALL of that would fit in the water of World Showcase Lagoon at Epcot. Both are approx 40 acres. Google map WDW to get the size perspective.
Have fun!
Steve