The kayak is a quiet and efficient vessel for stalking fish. I have had trout and bass rise to take something off the surface within a few feet of my kayak. It also allows you to get into shallow waters and places that are inaccessible to other craft. These are the kind of places that usually hold the fish you are seeking. Once you get into kayakfishing you will likely acquire more than one "yak" if you are serious about your yakfishing.
A 16' sit-on-top is a good choice on big lakes. It is able to handle the rougher conditions you may encounter and is also capable of moving you along quickly. In addition it is able to carry a lot of gear should you wish to camp as you fish.
The 12 foot sit-on-top is a good choice for use on moving water. it is long enough to track well yet short enough to negotiate quick turns on a winding river.
The other choice for me is a kayak that I can stand in. The ability to stand allows you to stretch your legs and gives you a good vantage point for sight fishing and casting and fishing with a flyrod.
I still have the original sit in kayak that I started with but compared to the modern sit-on-top kayaks it is uncomfortable to fish from and it hasn't moved much since I got my Wilderness Tarpon 120 kayak. My canoe has hardly seen the light of day since I discovered the much more efficient kayak. I would still enjoy "canoe-tripping" with a buddy but I haven't given up on trying to figure out how to portage my kayak. The portaging and carrying factors are the weakness of the kayak.
Stand Up Yak