I would say all the advice on here is very well said.
I would add a couple pieces of advice:
1) Whether it be a canoe OR kayak (sit on top or otherwise) good paddling knowledge and first hand experience of how your craft responds to various conditions beats out how "good" the craft is any day. I know an individual that won't even sit in a kayak because he is so well seasoned in a canoe that he canoes with a whitewater solo canoe with a deck cover and swears it is better than a kayak.
2) As someone said, try out the type of craft you are interested in before buying one. You may find that you (or your wife) don't like how the stability of a craft feels. With canoes there are various levels of "stability" and canoes are considered to have a 1st, 2nd, and 3rd level of stability, meaning some only have 1 and 2 (which is fine) it's just that you don't "feel" stable, even when it would take a serious wave to tip you over.
3) Once you decide on a craft, get out and use it in a "controlled" environment prior to taking it into the "wild" as many things can and do happen. Whether it is a canoe, kayak, sit on top kayak, or tandem, you (and your wife) will need to know how to correct or respond to many conditions that the water can expectedly and unexpectedly throw at you.
4) For example, while a sit on top may not seem more stable, in the event you flip, if you and your wife are in it, you both would be ejected from the craft and you both would have to get it righted and back in (possibly in the middle of a lake, river, class 1 water, whatever. If you are in a tandem kayak (not sit on top) you and your wife should know exactly how you need to "roll" it back over TOGETHER. This can be tricky (especially in the middle of a lake where you or your wife could be panicking, gear floating away, etc.
5) Take a class from a local outdoor store (many around here offer "lessons" on how to paddle, handle different situations, etc) It helps to know what type of paddling works in different situations, and how to right a canoe/ kayak in the middle of a lake where you can't touch bottom. Believe it or not, this is a basic skill taught at our summer camps for kids around here in the Midwest, and I've personally seen many kids/ young adults panic when it actually happens, even when they know what to do. There are some "tricks" to knowing what to do in these situations.
6). I'm reiterating this one:
Once you've decided on a craft take it out and use it many times before you get it out in the wild! Especially if you're not experienced with that particular craft. First hand experience of your craft and experience using it goes a LONG way when you know how it responds to getting hit by different size waves, winds, eddys, and if you're in a craft that you are paddling with your wife, even if you've been married a long time, know how to communicate back and forth in a craft your "driving" together, who is doing what, when, and what that does to the navigation of your craft. Just like in a marriage, communication with your paddling partner is critical!
on a side note, my wife and I can't do quite a few other things together very well, but paddling a canoe is one of the few things we just click on and we know exactly how to communicate with one another to "drive" our canoe. On another "side note" I go to the Boundary Waters Canoe Area Wilderness, every year for the last 5 now (going on my 6th trip this summer). It is backcountry canoeing/ camping where you are out of touch with civilization (unless you take a satellite phone). If something happens up there it is very hard to get out if you or your paddling partner are critically injured, it's a LONG way out in most cases, well at least as far as we go in. A very seasoned young paddler drowned in a very deep lake while fishing from shore late one evening, It took a week of specialized equipment to get in find the young lad's body, and get it transported out. Not saying that to scare you, it is leading me to my next point:
7) ALWAYS wear a life jacket when paddling! It seems like "common" sense, but when the weather is really nice, it is tempting to take off the life jacket. In fact, when in the boundary waters, I wear it even on shore, as some shore lines drop off fast an rocks can get slippery. I even use my life jacket as a secondary pillow when camping up there in the BW, as winds have been known to blow a tent into lakes with people in them.
No matter how good a swimmer you are, try swimming any distance with shoes and clothes on, with and without a life jacket. It is amazing how much certain clothes and boots/ shoes can hamper your ability to swim (and survive) under the unfortunate circumstance you do end up in the drink!
Sorry for all the "negative", but when it comes to paddling, it is important to know your abilities, and know your risks, and be well prepared for knowing what your skills and abilities can and can't do to help you get out of certain situations.
9) Also, practice putting whatever craft you get onto your vehicle or mounting system. This can be easy or not, and make sure that you and your wife can do it together, or depending on the craft, alone. With my lifted liberty I can actually load my 85lb canoe better by myself than with my wife as it is hard for her to lift it up high enough to get it on the JEEP (and she's 5'10").
10) I personally also prefer a canoe, you can do a LOT of paddling in a canoe that you can do in a kayak. We have a kid, two dogs, and it is nice taking them all, when paddling it can hold a lot of gear (even if you can fit everything for two weeks into a backpack
) Beyond this trip, a lot of people I know (including myself) often find a canoe to be more versatile.
11) Get a dry bag! In the event you do go in the drink, you want your stuff to stay dry and float
Nothing stinks more than getting dunked in cold water, trying to dry off, and have to spend a day (or more depending on your weather) drying your clothes and sleeping bag out, trust me, I know
Good paddles, canoes, and crafts are one thing, being well versed in how they handle can go a long way! Just like a good JEEP
on the paddle note, I have one friend who prefers a 60" boat oar over a dedicated paddling paddle. In a canoe on a bigger body of water, the oar gives a LOT of control. Obviously if you're leaning towards a kayak, you'll want a good kayak paddle...
I'm sure I could rattle off much more, if you want to hear and see some awesome paddling stories and advice go to
http://www.bwca.com It is a site that is dedicated to the Boundary Waters, but many people post about trips paddling other places. Lots of great tripping while paddling advice, knowledge, and what not.