It's not only about sensitivity.
First of all microphones with 1/2" capsule are more expensive than ones with 1/4". Obviously it is not always a rule, but the bigger area of the diaphragm the more sensitive microphone is. Additionally size dictates its mechanical properties (resonance frequency, frequency range, etc.). But is sensitivity so important?
Sometimes we don't want our microphones to be so sensitive, especially when you want to measure things like gunshots or jet engines. In that case you need less sensitive capsule, such as B&K 4941, which is 1/4" and has sensitivity of 0.09 mv/Pa! This ensures you will get 184 dB of maximum SPL - it is a lot! Mind that maximum SPL for sound wave at standard atmospheric pressure is ~194 dB, above there is a shockwave. And believe me - this microphone is way more expensive than some with 50 mv/Pa in their specification.
Therefore thing you should consider on first place, is to choose capsule with least Residual Noise Floor. This will minimise any noise in your measurements and affect the Dynamic Range of measurement kit. Small curio: you can even buy microphones with very high sensitivity (1.1V/Pa) that are allowing you to measure the sound levels even down to -2dBA! G.R.A.S. 40HF could be an example - it has 1 inch membrane. You can imagine how expensive is this microphone...
Just to wrap it up. I suggest you to focus mainly on two things.
If you are not after measuring high SPL, then get the microphone with decent sensitivity and least possible noise floor. Also do not forget about it's frequency characteristic - you want it to be as flat as possible.
Please also pay attention to built quality of your microphone and other possible applications. For example you can buy ones with XLR connection and 48V phantom supply (instead of 200V and LEMO connector) - such microphone can be used also with normal USB audio interface as measurement mic.
Please also remember that meter is very important as it can provide automatic scaling of dynamic range and ensures the bandwidth of your measurement. Plenty of low budget SLM's allow you only to measure up to 8kHz, and you must switch between ranges. What's more, when you switch between low and high it produces different results...