It's been demonstrated many times that C ratings mean nothing. They can be useful for comparing batteries within a brand, but there are not established standards for performance and C ratings. One brand's 25C pack will outperform another's 45C pack. If you want to get into the actual quality of packs and their performance, the only thing that actually matters is internal resistance. There are ways of checking IR, which is a handy way to not only see which batteries are actually the best but also see how well they age.
What's actually important for the OP's situation though is flight time. If his plane draws an average of 30 amps during the flight, a 3000 mah battery will be dead flat is just under 10 minutes while a 3500 will allow a small margin of error. If a pilot is ok with shorter flight times, he can go down to a smaller battery. EDF pilots routinely accept 3 minute flight times because they want crazy amounts of speed but have to keep the weight down. IR numbers are going to really matter to them. To an Apprentice pilot, not so much.