I'm confused between 2 low budget recording microphones. One has a sensitivity of 2mV/Pa and the other one has that of 2.5mV/Pa. Which one should be chosen? How does sensitivity affect sound quality?
If your preamps are your limiting quality factor, higher sensitivity will result in better recordings. If your microphones are your limiting factor, the lowest equivalent noise level will make the better recordings.
All other things being equal, which they never are. So a difference like that is rarely going to be decisive.
Also, low budget microphones (and I mean low budget) tend to fantasize their specs anyway because if their makers had the required measurement equipment and location, they'd also have what it takes to develop non-low-budget microphones. And they won't outsource the measurement since the results would make them sell worse than if they just invented values.
That difference in sensitivity isn't really going to make a difference in recording quality. What about other factors, such as the frequency response, frequency range, whether the mic is directional, etc. All these are of more importance depending on what sort of recordings are envisaged. I understand that this is off axis to the direct question, although those sensitivities mentioned seem to be possibly the smallest consideration when choosing a mic.
Many other factors are of more importance, including the kind of mic chosen - ribbon, dynamic, condenser, etc., not forgetting the rest of the recording chain, making a difference of 0.5mV/Pa not so important.
And it's unlikely you'll find two mics with every other spec identical, except that sensitivity.
Every piece of equipment in a recording set-up affects the final product, that said, you can often achieve good results with mediocre equipment if you're willing to put in the time to find out what works. Trial and error are often part of the creative process, but it is usually time consuming. specifications don't mean that much if your focus is not on design and engineering. how it sounds when used in conjunction with the rest of the gear is usually much more important. Ask to try the mics in question for a short time and then purchase the mics that suit you best.