Having had six months of great fun in the K1 Challenger kayak 2012 (Blue model) paddling on canals and rivers all over the Southwest, I've just bought the 2013 (Green model) of the same kayak. Here's my comparison review:
First of all, both kayaks are excellent and easily the best inflatable kayaks you can buy for the money. A lot of thought has gone into the design of these boats. The 2013 model is in almost every respect an improvement on the 2012 model. It is lighter (but still very tough material), sleeker and has a lower profile than the 2012 model. It is therefore easier to paddle and moves more quickly through the water.
The seat is now detachable in the 2013 model with velcro attachments on the floor of the boat. The detachable seat is very comfortable, although I just inflate the back portion as inflating the bottom portion made me feel like I was sitting too high in the boat. This doesn't detract from the comfort of the seating. Having said this, I found the seating of the 2012 boat also very comfortable. Some reviewers found that the attached seat began to rip on the 2012 model, but this never happened to me.
The greatest improvement of the 2013 model over the 2012 model is in the new, much larger, detachable skeg. The attached smaller skegs on the 2012 model also work very well, but the detachable skeg on the 2013 model works superbly well keeping the kayak in a straight line and making it easier to paddle. It is difficult to take on and off though and you can't fold the boat up with the skeg left on. The trick is to take the skeg on and off when the boat is deflated although it is still very stiff even then.
So the 2013 model is nearly all improvements on the 2012 model. However, there are two particular flaws in the design of the 2013 model that need to be considered when deciding between the 2013 and the 2012 model. There is a separate inflatable portion that you put inside the front of the kayak for both models. This is an excellent and crucial design feature as it gives you something to push your feet against when paddling like the foot pegs in a hard shell kayak. This inflatable portion is a perfect size in the 2012 model. However, in the new 2013 model it's far too small and my feet couldn't even reach it! I am quite small (5'6") but even so this is a strange error from the designers of the 2013 model, especially as they got it absolutely right on the 2012 model. I've therefore had to use the inflatable portion from my 2012 boat in my new 2013 boat - an option that won't be open to first time buyers of the 2013 model.
The second problem is that whilst the smaller size of the 2013 model is great for me being 5'6" and of slim build, it is still a fairly tight fit even for me. I would suggest that someone of a much larger build would find it very tight indeed and would want to consider the larger opening of the 2012 model which has far more room.
The included paddles on both the 2012 and 2013 models are both the same, and are both absolutely useless! It's a shame that Intex has designed such a superb inflatable kayak with such hopeless paddles that will ruin the enjoyment of the kayak. You will need to invest in a basic asymmetric 2-piece kayak paddle (I bought a basic TNP paddle for around £40) to enjoy paddling the kayak for any reasonable distance.
The included pump with both models is the same and is excellent. It pumps up the kayak in a matter of a few minutes (you won't need to buy an electric pump which I made the mistake of doing first time around).
My conclusion is that the 2013 model is a substantial improvement on the 2012 model as long as you're not of a large or tall build. If you do decide to go for the 2012 model instead you won't be disappointed as both boats are very well designed and almost unbelievably good value for money. I tried out a £600 hard shell kayak recently and decided that I much preferred my £60 Intex inflatable!