IIRC the general consesus it that the passanger airbag doesn't really help as, due to the geometry of the interior, it's too far away from the passenger to be effective. Unlike a standard tin-top (where the passanger is substantially nearer to the dashboard. It's hard to see how you are getting anywhere near it with sealtbelt pre-tensioner.
IMHO if you get the SRS airbag blank instead - it's also one less thing to cause the SRS warning light to go on.
Just my personal opinion