Since this is your first trip, take the SLR. The memories of your first experiences with Disney Magic will be all the more immediate with the higher quality of the SLR, and of course the SLR is more capable and flexible (able to get the shot in a wider variety of conditions) than a P&S.
But you can also take the P&S and let your significant other carry it, or even trade back and forth a few time. That way you both have cameras and can get pics even when you are sepparated.
Carrying an SLR around the parks all day CAN be cumbersome and tiring, unless you take some precautions ahead of time:
1) Pare it down! Many SLR users carry a bag full of lenses, filters, flash, batteries, memory, and a few kitchen and bathroom sinks. This gets old, FAST. So I pare it down - I carry no bag (speeds entry into the parks since I can skip the bag check), and I carry a spare memory card or two in a pouch on my camera strap, one spare battery, and a small external flash unit that fits in my pocket (Canon Speedlite 220EX).
Most importantly, I got myself a good walkaround lens that has a range of zoom. Actually, I have two - the 18-55mm Canon kit lens that came with my camera (yours should have come with the same lens, and it's a very nice lens), and I later bought a Sigma 18-125mm lens. 18mm is great for wide angle shots - and you WILL use it at WDW! - while 55mm is a good moderate zoom, and the 125mm Sigma is more of a 6x zoom. I have used both lenses on my WDW trips, and by having a single lens that has a wider range of zoom, I have no need to carry any spare lenses around with me.
2) Spend the money on a really, really good strap. The straps that come with Canon SLRs are nice, but they are far too thin and uncomfortable for all-day use. You need a really WIDE strap to distribute the weight of the camera more evenly, and you need PADDING to prevent the strap from digging into your neck, and you need a strap with LENGTH so that you can loop it over your head and shoulder (which takes the weight off your neck).
I have actually abandoned regular straps entirely and made myself a camera harness. Yeah, it looks dumb, but it takes the weight of my camera completely of my neck (important for me since I have herniated disks) and puts the weight all on my shoulders. The harness has enough slack to allow me to shoot both horizontal and vertical, yet holds the camera comfortably on my lower chestwhile I walk, even when I don't have my hands on the camera. I even added pouches to it for two spare memory cards, placing them in easy reach instead of getting lost in my pocket (this idea comes from the decades-old practice of elastic film canister loops on camera straps).
3) USE IT! A camera is nothing but an expensive millstone around your neck if you put the lens cap on and only whip it up to take a pic once per hour. Leave the cap off (unless you're on a water ride) and leave the camera turned on all the time, and take more pics. The more you use it, the less it will seem like somthing you are carrying - it will seem more like something you are using, and the weight and bulk will no longer bother you.