Sit in side kayaks are generally faster, track a better line and offer more versatility. But don't go shorter than 12 ft and 14 ft advised unless you want a whitewater boat.
SOT BOATS are generally "more stable" but ths makes them considerably slower, harder to get up to speed and with less glide. Sit inside kayaks are also very stable depending on hull shape, width but in general because your center of gravity is at waterline instead of above waterline. So I order for a SOT to be as stable as a sit inside boat, the SOT is generally made wider and with greater volume. This necessarily makes the wider SOT fatter and slower.
In terms of versatility, I can pack my WS Pungo 140 with enough dry gear and food to camp for few days. And when she is loaded properly, she is a dream boat with lots of to speed and glide. A SOT with its higher volume, higher profile and much wider beam would catch wind, is harder to paddle straight and dry storage is limited.
If you are looking for a boat that you can paddle out a mile or two, get in and out of a lot, fish and paddle wiggle waggle back to the ramp, then a SOT is your choice. But if you want a boat that will take you 10-20 miles, fully loaded, camp or picnic, fish and do it rapidly, then the sit inside kayaks are your choice.
BUT, as mentioned earlier, hull shape, chine, length, paddeler's skill should all be considered before buying.
I like the WS 140 and my wife paddles the 120. We love these boats from the big waters of Teton Lake to wisconsin river reaches north and south, the Chippewa and slower flat portions of the Flambeau, devils lake, lake wisconsin a our home waters at Busse lake......I would not take these in any white water more than level 2. They are too long with too much chine.
For friends, I have an Old Town 138 that has a double hull and a very flat bottom like a canoe, wide cockpit and is therefore very stable. But it is wide at the waterline and is therefore by definition harder to get up to speed, had less glide ratio and is slower. But a very nice classic kayak with stability as it's hallmark. And it is an I breakable tank against rocks and logs in current.
I have no interest in narrow, less stable truly fast kayaks as I don't want to trade the comfort for performance. Similarly, I personally have no interest in a wider, flat bottom, short SOT as I don't want to trade still more stability for speed. How much stability do you need? How much speed do you want?
They're are plenty of paddeler's who would say that my WS 140 is a big fat slow bathtub of a boat. But for me, it is prefect for my uses.
Any boat you get should look beyond cost, to a point, with attention to what you want your boat for. And if you don't go over the top on any one aspect, you can really start to narrow down the many many choices. If you go over the top with stability, you can find some speed. If you don't go over the top on speed, you can add some comfort, storage and stability.
I would suggest getting a copy of Canoe and Kayak magazine and go to all of the manufacturers websites.