simple maths mate...

3bar map sensor reads 3bar absolute (so 2bar boost, 30ish psi)
2bar map sensor reads 2bar absolute (so 1bar boost, 15ish psi)
2.5bar map sensor reads 2.5bar absolute (so 1.5bar bost, 22ish psi)

if you're intending to run 6psi boost it seems stupid to use a 3bar sensor...

...but if you're intending to run 26psi then it seems stupid to use a 2bar sensor (altho it can be done with trickery)

i'd say if you wanna run 22psi you may aswell run a 3bar, as you may strugle with having an overbost function on a 2.5bar. also 3bar chips are easier to get hold of off the shelf, and the sensors are easier to get hold of