k5 better af, better build, weather sealed, slightly better iso,
i would say body, as you will never use f2.8 for motorsport. and the 55-300mm should be fine for motorsport, ok the 70-200mm will af faster but i think the af speed on the k5 will be more than enough.
I have been using a Pentax-M 200 F4 for motorsports on my K7 been happy with it. the sigma as scottthehat said is more expensive then you need.
I have the tamron 70 - 300 not really all that happy with the IQ but think i would have the same issue with the Pentax 55-300, been spoiled by primes. But the general consensus is the 55-300 is a great lens for a consumer zoom. lots of examples on pentax orientated forums.
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