Remember that if you are doing lots of solo paddling, there's something reassuring about a sit on top (an SOT, e.g. Tarpon 120). If you get capsized, you just hop back on. For sit-inside kayaks, there can be a less-fun process (unless you learn to roll) of getting back in, getting a spray skirt back on (if using one), pumping out water, etc. Sea kayaks, especially as they get longer (16-18') are meant to cover ground efficiently. There's always a tradeoff between weight, stability, ease of turning, speed, etc. in kayaks.
I don't like rudders on a sea kayak, but I love my retractable skeg (rudders help you turn, skegs help you go straight). They even put them on smaller boats, like a Remix XP10, etc. They can help a more easily turned boat track better on flat water.
If you are into fishing, a SOT is the way to go. Kayak fishing is really popular, and more and more manufacturers are getting into (and/or carried away with) making angler specific boats.
If I were you, and just getting started, I'd suggest looking at a 12' SOT (like a Tarpon 120, etc.) or a 15-16' sea kayak, depending on how you want to use it. Craigslist is the place to go for sure. If you get into kayaking, you will definitely end up with a few boats!