Realisticly, inside the water, for most paddlers, plastic vrs composites is too overrated, kind of like bike material -AL-CroMo-Ti-CF... a lot of boat feel and speed is the design over the material. Durability asside, the big advantage of composites is their strength to weight, i.e. you can use less material to make a kayak hull strong which makes the overall weight less. So, outside the water, the composite kayak is much easier to portage, load/unload etc..
In the water, I am the believer that a strong paddler in a plastic kayak will easily be faster than a weak paddler in a composite, even in a lesser design. It also is true that 95% of the recreational kayaks are plastic... and rec kayaks are designed different that full-on touring kayaks which will come in a mixture of plastic and composite designs...
If you have a good deal on a tempest 170 and don't mind dealing with the weight out of the water, I say go for it. Of course, Darren is correct that paddling it will be the only way to say if a boat is right for you.
Jay