Light and DOF
25mm 1.4 <=> 50mm 2.8 (Full Frame) in terms of DOF.
In term of light what you say is not correct: gather the same light ? No, both sensors do not collect the same quantity of light in fact. They will have the same exposure (the same density of light will fall on sensors) but it will fall on an area 4 times bigger on sensor A. The quality at the same ISO will be much less noisy with sensor B
Larger sensors tend to have lower noise and higher dynamic range.
Read the article I posted above.