Either should be good, but you are right to ask about the C ratings. Ideally, you want a battery that can provide at least 80-100amps continuous, so if you take that trueRC battery:
5000mah x 10c = 50amps continuous, which isnt too bad, but somewhat below par for waht you want really.
If you go for a zippy, then a 20c 4500mah battery will provide 90amps continuous, which is much more like it
I think the main difference between the zippies is the C ratings, and their shape, hence the different letters (K, R, S etc); my advice is to buy the best capacity & C rating you can afford, provided it fits your truck ofcourse.