As has been mentioned the C rating and capacity are linked so it's worth always trying to buy batteries with comparable capacity and rating, or better stil buy two or three packs that are identical as that will keep performance consistent from one race to the next.
I know my Corally 120c have far greater punch compared to my 80c Vampires, but also know that a fellow racer thought my Vampire 4600mah 80c shorty had more punch than his 5000mah 100c Orion, so I don't believe all batteries are made equally. Additionally how you charge and store them may have an effect on life span and performance.
I run four packs of Corally Voltax in my 2wd and normally put in around 1000 to 1500 after a five minute race.
Personally I'd buy the highest capacity, highest C rating batteries you can afford as they will give you greater performance over a longer period of time and will save you wanting to upgrade should you find you want some more power as your ability improves. Check out the Corally batteries
here
Also consider whether they are BRCA legal, some clubs won't mind, others will and if you want to take part in regionals or nationals in a year or two they'll have to be on the BRCA electric board list.