Interest upfront has the below advantages:
1) You are not subjected to DIRT rates on maturity which may be higher.
2) Your DIRT eligibility is assessed at account opening stage which will suit some people.
3) You have use of funds straight away. For some people, who want capital preservation, but need income for expenditure, this can act as a budgeting tool.
Interest upfront has the below disadvantages:
1) Due to the fact that you get the interest up front, the return is not as great, as one does not get compound interest. (Brendan's point)
2) Conversely, your DIRT eligibility is assessed at account opening stage which will not suit some people. For example, if you are aged 64 now and aged 65 at account maturity stage.
So, is interest up front worth doing?
The key aspect of the decision, for most people, will be expectations for DIRT rates. If you think DIRT will go up, then it is worth going for interest up front. If you think DIRT will stay the same, then you may be better off waiting for interest at maturity date.