They would both be great cruises, and you'll understand that once you do a Disney cruise and are hooked like most of us. I just think the Fantasy going to Castaway is going to be a better introduction to cruises and be fun from start to finish. Bonus: you can also hit up WDW for a day or two on either side of the cruise.
I'm going to endorse this answer. I love the Magic. I've sailed on the Magic 8 or 9 times. Believe me, we LOVE the Magic. But I've done the Canada cruise in October (once with Disney, once with Royal Caribbean). I gotta say, it didn't wow me, to be honest. My trip report for Canada is listed in my "signature" below if you want to read it. And as everybody else noted, it's not a warm vacation, if that matters to you.
http://forums.wdwmagic.com/threads/magic-al-cruise-to-canada-8-5-12.843771/
You'll love the Dream as well. It has a ton of very cool features (including the "Beauty and the Beast" live show and "The Golden Mickeys", which is DCL's best show ever). It'll be warm, you'll get Castaway Cay, you can hit WDW before or after for a day or two. So for all those reasons, I would probably suggest the Dream for this particular choice.
BUT-- while you're onboard, make sure you make a future reservation and then check out other Magic itineraries. You can take the Magic from NYC and go south, or you can get the Magic in Miami (like I'm doing next April). ORRRRRRRRR -- you can do the Magic in Europe! Now THAT was a great cruise.
2. New York/Canada are pretty chilly in October. You won’t get much use from the pools, whereas the Bahamas will be a great temperature for active adventures like snorkeling, beaches, pools, etc.
4. Port Canaveral is their home port, and as such they have their own private terminal. I’ve sailed from other ports and it loses a bit of the magic compared to Canaveral.
#2) The pools are heated to 82 degrees, so you can use the pools every day if you want. Believe me, there were people in the pools in ALASKA! Don't let this be the thing that discourages you. Yes, it can be very cold on an October Canadian cruise, but you can still swim and enjoy the hot tubs if you want.
#4) Completely valid point. Getting on a cruise in NYC is -- in a word -- uninspiring. It's a huge nondescript warehouse-type atmosphere. DCL tries to brighten it up with some posters and banners, but it's not particularly attractive. Port Canaveral, on the other is beautiful, with amazing tile work and a map of the Caribbean that spans most of the floor. Disney videos play on TVs for the kids and I'm pretty sure it's less than $40/day to park there.