A lot of first-time kayak buyers seem a little ruffled at first when they first learn the kayak they are interested in doesn't come with a wheelcart as standard, but there is good reason that they don't. No one wants to make assumptions about how and where you're going to be using your kayak and that may very well dictate which wheelcart is the best solution for your yak.
There's a reason that there are various models to choose from and as usual its all about horses for courses. Some wheelcarts are better for certain applications than others - it all depends on the kayak hull shape and design as well as the usage scenario, ie: where the kayak is being used and carted and how much weight it is carrying.
There are basically two types of carts available - plug in carts that are inserted into scupper holes, or sit-on carts, which are strapped onto the hull. The latter are typically more likely to fit a wider range of hull types (kayaks and canoes) and are the only solution for boats that don't have scuppers that can be used for a plug in cart. Lets examine these first.
A much more versatile version of the sit-on cart is the C-Tug cart made by Johnsons Outdoors. This fully collapsable cart is durable, reliable, works well with a much wider variety of hull shapes (due to it's wide-base pivoting support pads) and can even be modified for use as a camping stool. The wheels roll well over a variety of terrain types and the frame is very strong. At around AUD $179, it's more expensive than the previously mentioned folding cart, but it's definitely the superior product. There are 2 types available, one using a pneumatic wheel, another using a wider plastic wheel, the latter of which is said to perform better in sandy conditions. If there is a down side to the C-Tugs, it's that they tend to get pretty squeeky and require regular lubrication to prevent it.
Hobie also have a sit-on cart offering and this one is equally as strong as the C-Tug (perhaps more so) and almost as versatile. Due to it's padded supports that run along the length of the hull (instead of across the hull, like the folding cart) it will better fit a wider range of hulls. Unlike the C-Tug, it can't be broken down for in-hull storage and its also more expensive than the C-Tug though, weighing in at around AUD $230. But it certainly looks the toughest option of the lot.
Sit-on carts are really the only option for all canoes and many kayaks but without doubt, a well suited plug-in cart is easier to use if your kayak has scupper holes that can double as plug-in receivers for a wheelcart. This is because not only are they more effective at carting your kayak around without having the hull move around on the cart frame (which is more likely to occur with sit-on carts), they are also simpler and faster to put in and take out. The ability to insert or remove cart quickly can be a big advantage when landing or launching in any kind of wave activity. So if your kayak supports it, I'd generally recommend a plug-in cart. They're not all built equal, however.
Another alternative that works for smaller, lighter and or thinner kayaks are the mini wheelcarts, which are limited in what kind of kayak shapes and weights they can be used with, due the sharp V shape of the cradle. Wheels that are hard and as small as these are also only going to be of any use on hard terrain, and will not be a good option for beach sand.
Hobie also have a couple of much much larger Dolly wheel carts available and these have been designed specifically for their Tandem Island trimaran and are offered as an alternative to their plug-in carts (discussed next page). These are recommended for users who have have experienced problems using plug-in carts, and or are having to cart their TI's over long distances, especially long beaches. They are used in much the same way as any sit-on-top cart (hull is rested on cradles, webbing straps secure hull in place) but present their own challenge in their weight and size. While relatively safe and easy to use, they are large and heavy and cannot be stowed on the boat when in use. For some users that will be a problem, for others it won't be.
There are numerous versions of sit-on style kayak and canoe wheelcarts available, the above mentioned being a few of the more commonplace. Perhaps the most creative sit-on style wheelcart ever devised is the Batcart, designed by HawaiiKayakSailing to load, carry and launch trimaran kayaks (this trimaran kayak to be specific).