It depends on your tax (and bond situation). If you are a 40% tax payer, you effectively get 4 000 back a few months after the tax year for every R10 000 that you put into your RA - that's if you havent' used up your tax limit of only 7.5% of your pensionable salary by contributing to a company pension fund. If you are a contributing employee and you have used up your limit, you basically enjoy no further tax relief. BUT if you are self-employed or employed but not contributing to a pension fund, you should get the R4 000 for every R10 000 (just keep in mind legislation could change at any time). That's a 40% return on your money (for the first year/the tax year only).
On top of that, your investment is also exposed to the equity, property and bond markets (depending which type of underlying fund/portfolio you choose) and the potential growth that they could give you over the few decades to your retirment.
If you choose an aggressive portfolio (Allan Gray allows you at this stage to invest more than 75% in equity/shares), you could end up earning a return over the next 20 years very close to the interest you're paying on your bond. BUT there are no guarantees around the returns on your RA, only historical data (about a 100 years) and we don't know if the patterns will be repeated in future. On the other hand, if you choose to pay off your bond, you have a reasonably good idea what % of interest you'll be saving over the next year - much more certainty.
As Alf101 points out, an RA's biggest benefit is that it enforces the disclipline of staying invested until you're 55 and the certainty that you will have some income when you retire.
As you may pick up, there are many intricacies around the decision whether to save via an RA or rather pay off your bond first. Best to speak to an independent, non-commission driven financial adviser before you actually decide on an RA. Unfortunately, I'm not one and cannot help with the finer detail.