If u invest in a SIPP the government adds to your contributions.

Under current tax law, when u access your SIPP 25% can be withdrawn tax free and the balance is taxed at your current rate.

With an ISA all withdrawals are free of tax.

So if u have a longer term frame and u can compound your contributions, a SIPP could be the better option.

As u get nearer to accessing the funds, current pension age 55, it may be better to contribute some funds to an ISA, which could act as an emergency fund or the dividends could be withdrawn, to supplement your income.