I can't say which is better value. Apart from value, there is the question of what you want to achieve. Purchasing notional years will mainly benefit you if you are aiming to increase your guaranteed pension income for life after retirement. If you survive a long time this could be very good value. If not, it won't - although there may be a peace of mind benefit in the meantime. Notional years will also give you a marginal increase in your tax free lump sum, depending on the number of years purchased. You can work out approximately how much you will gain with notional years by playing around with the Pension Modeller here :
http://www.cspensions.gov.ie/calculators.asp .
One major advantage of AVCs is that it allows you to maximise your tax free lump sum at retirement. With 28.5 years service your tax free lump sum in your pension will be 85.5/120 * pensionable salary. However, Revenue allow you to take 120/80 * pensionable salary. So you could use the money in the AVC to increase your tax free lump sum by 34.5/120 * pensionable salary. Any balance would usually go to an ARF/AMRF.
With 18 years service I am assuming you are a pre -2004 public servant. If post 2004 you would be considerably disadvantaged by retiring at 60. Also, if pre 2004 you may be entitled to a Supplementary Pension between retirement and State Pension age.
Two other thoughts. 18 years service suggests you joined at 31. Do you have any service prior to this which wasn't pensionable at the time, eg temporary, probationary, etc. If so this can be bought back now. If available, this is considerably cheaper than notional years and is certainly good value.
Finally, notional years and AVCs are not necessarily mutually exclusive, provided you don't exceed Revenue's annual tax relief contribution limits.