Yes - 10 more Cs costs 20 more cents. What plane is this going in?
Just remember bigger is not always better; if your power system doesnt require anywhere near 40C capacity, go with the 30C. Not only is it 20 cents cheaper, higher C ratings typically mean heavier batteries, all else being equal.
I use 20c packs for the most part.
Aeryck is hitting at what I was really asking. This is for a axn floater jet with stock motor and exceed RC proton 30a esc. Given that, is there a reason I would not pay 20cents for the 40's? Weight looks to be 3 gram difference.
I use 20c packs too. My power system draws 30.5A at WOT. 2200mAh pack at 20C means the pack is good to about 44A give or take. Some packs (read: Turnigy) may say 30C but dont really like much over 20-25. YMMV. Its good to overshoot a bit but if you have a 30A power system (basing off your chosen ESC), a 2200 30C should be plenty.
Also remember when using dual batteries (obviously not what you are doing) in parallel, you can essentially drop your C requirement by 1/2
Higher C batteries tend to have a flatter discharge curve. This could be a good thing or a bad thing depending on your application. If the voltage stays flat and suddenly drops off instead of slowly dropping for the whole flight, you might find yourself running out of power unexpectedly.