Denoising approach for a combination of several ADC voltage channles
According to your processing chains (filter --> sum vs sum --> filter) the two approaches, by being LTI systems, should produce the same results under infinite precision arithmetic. You can show this mathematically by using the properties of convolution operation.

Note that a moving average filter is just an ordinary LTI system.

Note also that nonlinearities such as quantization operations may affect the above fact's validity.