As things stand you would be better off contributing to your employer's PRSA rather than going it alone.
You will get PAYE relief at the top rate of tax no matter what you do. But you will only get PRSI relief by contributing to your employer's PRSA. The government is working on ways of granting PRSI relief to people to go it alone but according to , the rules are not yet in place.
It's also worth remembering that your employer gets full PRSI relief on all PRSA contributions, both the ones you make and the ones s/he makes. So if a big chunk of your pay rise is going to be a PRSA contribution, your emplyer can afford to be a lot more generous than if the money was going into your paypacket.