C ratings are in many ways just marketing tools, should always be taken with a grain of salt.
Having said that, in our application we run about 8-10C, so 25C batteries are plenty.
Nothing really gained going to higher C, and higher C batteries are typically heavier than lower C ones. They have better power density, nut lower energy density.

That is why many use the TP 25C packs.

Pat MacKenzie

Based on what I have learned flying electric RC, the "C" rating multiplied by the milliamp rating tells you (in general) how many amps can be drawn from that particular battery.  For instance a 3,000 milliamp (which is 3.0 amps) with a 25C rating can power a motor that draws 75 amps: 3.0 amps x 25C = 75 amps.   A 3,000 milliamp (3.0 amps) battery with a 35C rating can power a motor that draws 105 amps (3.0 x 35= 105).  So, if your motor draws 90 amps at the power setting you are using in flight, a 3,000 milliamp, 25C battery would be overtaxed trying to feed your motor because that battery was designed to provide a draw of 75 amps.

The manufacturer of the motor should tell you how many maximum continuous amps the motor will draw, or you can use a power meter (watt meter) to determine how many amps your motor actually draws at a given rpm setting.  Your motor will draw more amps in a static test, than in flight, but that just gives a little safety margin.  Of course, if you use a watt meter, your reading will only be good for the prop you are using for the test.    Different props can draw a different amount of amps from the motor.    Once you know how many amps your motor will draw in a static test (or you could use the motor's max amp rating which is probably overly cautious), you can calculate what the minimum "C" rating your battery should be for a given milliamp rating.   

I guess I should add that the formula (Milliamp rating x C rating = amperage draw the battery was designed to give) stays the same regardless of how many cells the battery has.

So, Ken, your 6S 2,700 35C battery should be capable of delivering up to 94.5 amps to the motor (2.7 x 35C = 94.5).   And your 4S 2,800 35 C battery should be capable of delivering up to 98 amps to the motor.     If you bought a 6S 2,700 25C battery it should be capable of delivering up to 67.5 amps to the motor (2.7 x 25C = 67.5 amps). And if you bought a 4S 2,800 25C battery, it should be capable of delivering 70 amps to your motor (2.8 x 25C = 70 amps).  Your 35C batteries may have been a good deal of overkill and 25C batteries might be quite adequate.   It all depends on how many amps your motor draws. 

Here's a thought.   If you are using an ESC rated at 70 amps or less, and the ESC is not hot when you land that's a pretty good indication the motor is drawing fewer than 70 amps and therefore 25C batteries should be adequate.

Joe Ed Pederson