We're also a family of 5, boy 6, girls 8 & 10, from Halifax.
We're big fans of off-site. We stay in a timeshare on I-drive, but there are plenty of house rentals, unused timeshares, etc. to choose among. We've use RCI a couple of times, they sometimes have a 2-BR (full kitchen, dining room, living room, laundry, privacy for parents) for $350/wk. Our kids loved the pool and on-site arts and crafts. Way more relaxing than cramming 5 into a hotel room, and the on-site choices for 5 are much more limited than for 4.
Rental cars seem pretty cheap to me, < $200/wk typical for a midsize which handles 3 kids (2 in boosters) - last time we had a PT Cruiser.
For flights, we flew from Halifax but we got 3 flights courtesy aeroplan. BTW, there is a great board just for aeroplan at FlyerTalk here:
http://www.flyertalk.com/forum/air-canada-aeroplan-375/
I'm very tempted to fly from Bangor; if you live near the U.S. seriously consider it. If you consider driving, there is also the autotrain from virginia, which is explained in a thread on this board in detail. Not sure it's great for Canadians, but I spent a lot of time looking at it - you save on a rental car, you do more driving but you can make that back sleeping on the train, the kids would love it... maybe someday.
Flights are usually our biggest cost, so we like to go less often, but stay longer (say, 2 weeks every 2 years save more air fare than 1 wk/yr).
Our biggest savings come from not eating in restaurants so often. We simply go to a grocery store and buy things the kids like that are easy to cook - frozen pizza, chicken nuggets, spaghetti, salad-kit-in-a-bag, cereal, etc. Suppertime means 1 parent takes the kids to the pool, the other heats/boils/unwraps the meal (and gets a minute to sit without the kids). Hey, it's not like the kids would eat much healthier in a restaurant (or if they do that it'd be cheaper). There's a big wal-mart supercenter near disney, just google it, they have groceries including a butcher, bakery, everything.
We eat out sometimes, last time we treated ourselves to the rainforest cafe in AK - it was a great mid-day break, a very hot day and that restaurant is very cool inside (almost cold). The kids favorite was golden corral (what did you expect? LOL). The loved to pick and choose, make their own sundaes, etc. I think it was about $8/each. My wife couldn't eat anything but roast pork and roast turkey, because EVERYTHING has MSG (she's allergic) - what did you expect? LOL
Although I budgeted more for buying snacks in-park, the kids actually didn't mind at all eating granola bars, etc. from the backpack - they figured out they wasted less time by eating while standing in line.
I invested the $20 for Tour Guide Mike, and I thought it was worth the money, especially if you're a 1st-timer. TGM has nice, detailed explanations of WHY the recommendations are. I found by reading them I had much better luck at improvising changes on the fly, and I never made a detailed plan or paper notes at all. But understanding HOW and WHY things are busy or not is much easier to remember. Also consider the unofficial guide, I never tried it but I think its more like $8.
My kids' favorite was the water parks, and they preferred TL over BB for the wave pool. Each park has a couple of areas, one for < 5yrs, 1 for 5-10yrs which they really liked. In fact, I think they liked the water parks at least as much as the regular ones, so give them serious consideration.
My closing argument: thinking only of money, staying onsite saves you a rental car (if you don't mind waiting for the bus), but you must eat in disney restaurants and stay in a hotel room. Staying offsite means a rental car, $10/day parking and cooking, but IMHO you get a lot more value.