A SIPP gives you full control over your investments with thousands of funds to choose from, while a workplace pension limits you to a shortlist picked by your employer.
Never leave employer contributions on the table. If your employer matches up to 5%, contribute at least that much to your workplace pension before putting anything in a SIPP.
Salary sacrifice through your workplace pension saves you National Insurance at 8%, which a SIPP cannot replicate. On a 50,000 salary with 5% contributions, that is an extra 200 per year.
The smartest strategy for most people is both: contribute enough to your workplace pension to capture the full employer match, then direct any extra into a low-cost SIPP with better fund options.
