Some thoughts:
Aluminum canoes are cold as sin when you're sitting in them. Also, if you're fishing they transmit the tiniest noise into the water very, very well. Also when you bang against the sides when you're paddling (occasionally we all get sloppy/lazy), it's loud and broadcasts across whatever lake you're on. Pros? They're cheap, can be fixed with a pop can and some duct tape easily (temp fix mind you...). They're also shiny and hard to be in if you have light sensitive eyes like me (what can I say I'm a delicate flower).
Plastic canoes are not very strong and usually have an inner frame in them to give them strength and shape. That inner frame is usually made out of aluminum tubing and they are hard to sit in on the floor and be comfortable. That doesn't happen often, but it might be something to think about. They aren't as cold to sit in, the reflection issue isn't there, but they aren't as strong if you move things about in them. They usually don't have nearly as many variations as far as shape is concerned. They are not as light as an aluminum canoe. Aesthetically they are ugly as sin...I actually prefer an aluminum canoe to a plastic one, all things considered.
Fiberglass canoes have a huge range of shape, sizes and features. You are also getting into a segment of the market that is more expensive than plastic/aluminum canoes. They are difficult to repair but are wonderful to sit in and easy to load. They are quiet in the water and a joy to paddle (I am a bit of a snob). The canoe I have is made by Navarro and has cherry ribs and is the Loon 16 model in red. It has cherry ribs with a fiberglass hull. It is beautiful to look at (and that matters to me somehow).
What is also important to consider is the type of water you'll paddle on most, lakes, rivers, etc... Also what do you plan to carry in it, how much, how long, etc... These variables will help you decide what shape you want your canoe to have. If you don't know these things and are a first-time canoe purchaser, then do yourself a favor and read some books, join some forums, rent some canoes and just get out. You'll find what you like, discover what your budget is and you'll be more informed about your purchase.
Another thing to consider, which IMO is as important as the canoe, is the paddle you will choose. There are a LOT of them, shapes vary, angles vary, price-points vary. Have fun looking at them.
PFD's are another thing to think about. I don't have any, realize it's probably not the choice most others would make, but that's me. I only canoe in flat water with no wind and that's that.
You will love canoeing.
