Discharge (C) is like the grade of the fuel, but has no effect on run time, Mah is how much fuel you have.
so more MAh will last longer no matter the C rating but will be heavier and if racing that would effect handling, and speed / acceleration.(For a basher you wont notice but Im a flyer (Quads) so every G matter to me)
A higher C will allow the battery to sag less underload and generally perform better, more punchy, however if a 30C battery can provide more amps than the motor will every draw there is little point in going to a higher C. (Higher C counts cost more and are more waight)
You are right in that there is a calc to work out a batterys amp's, so you will find a 40C 3000mah battery would actually have less amps than a 15C 10K battery so larger batteries you can drop the C rateing to what might sound low but really isnt...you need to calculate this though to work out what C you need, although factor in that most MFGs fudge these ratings anyways...so its best to get the best you can afford.
Use this to work the above out = http://www.kritikalmass.net/battery-calculator/
Also don't forget to check that the batteries fit your battery tray, and lastly, those batteries linked look like they are 2S, I dont have a sct 4x4 but it looks like it can handle a 3S so you'd get more speed by going that way than a higher C 2S, although I could be wrong in that it takes a 3S so make sure you cheack that, tbh though your battery linked are very well specced, if your not racing do you need that higher of a C rateing when you could save money on a cheaper pack?