You need to consider your own risk profile.
Taking a floating rate (variable) means your interest rate is ultimately linked to the rates set by government (usually prime or some deviation set using prime). Currently the rates are quite low for South Africa and an emerging market economy (its still high by first world standards). Taking a floating now could be worth while short term but a few bumps in the interest rate over the next few years could hurt.
Taking a fixed rate now means regardless of what the government do, you will pay one set value for interest over the agreed term (banks usually fix the first 10 of 30 years of a term, not the whole term, but you might be able to get 30 years fixed).
Best thing to do is to understand your risk appetite to this change, if you can handle it (interest rates might even go down, but unlikely) then go for floating but if you rather have piece of mind that you only owe 1 amount, stay fixed.
You can also get the two interest rates and Future value your bond taking a few variable scenarios into account. You can then stress test and see where is the point where floating becomes more expensive than fixed, and how high can it go before it becomes totally unaffordable.
