What he means by getting the largest C-rated pack you can, is that the higher the discharge C-rating, the better the pack performs, or rather, makes the truck perform. While the mAh rating is an indicator of how much charge a pack can hold, the C rating is an indicator of how fast the cells can discharge that energy. (That is not to say how fast they will run out) What you would notice between a 25C pack and a 50 or 60C pack is the feeling that the 50C pack is more powerful. Feels like it has more "oomph", ya know? That assumes the ESC and motor system can take advantage of that higher C rating.
Say you were using a 2S 25C Lipo in a car that had a 60 Amp motor system rating. If you put a 50C pack in that car, you might notice a little extra scoot. But if you ran that 25C in a car with a 120 Amp system, then ran the 50C pack, you would REALLY notice the difference. So, you kind of have to figure how much that higher C rating is worth to you and your car, cuz a 5000mah 50C pack is going to cost more than a 5000mah 25C pack. Can the car use that extra punch? then it may be worth it. It's not worth buying a 60C pack if your car and motor system can't take advantage of it.