It depends what your looking for, slow / holiday recreaction or long distance touring?
I have a perception "Swing" a great fun sit on kayak and even more fun in the surf, Im about 95 kg and get a wet backside on flat water, this makes it no good for touring as you would be constantly sitting in water.
The swing is a little slower than "conventional" kayaks as it is a little wider, so if you want speed go narrow beam (width) but its a little less stable than one with a wider beam.
Another suggestion is before you buy go and hire a few different makes / models to find the one you like best.

I bought mine as a factory second from Finn Kayak's in Perth. It's a sit in so not fit for what you want to do but factory seconds can save you some money and after a few scratches from normal wear and tear you can't pick the minor plastic weld job anyway. It saved me around $400. Make sure you get something designed for flat water with either a good keel or a rudder. Standard kayaks are designed for rivers and won't track well on flat water ie you'll be going around in circles all day.