While purchasing added years was once an attractive option (I did it myself), it is much less so nowadays, due the actuarial calculations having been tweaked. In its favour, it's a bird in the hand situation - you know what they cost, you pay for them and you know exactly what you're going to get from them when retirement time comes around. The AVC PRSA is less guaranteed but is much more flexible and, barring a complete disaster, will probably produce a better outcome.
Opinionated answer: For me, the AVC PRSA is the better option. Given the number of years to go before she'll be availing of the pension, you should probably consider selecting a mix of both medium and low risk funds.