If your workplace pension is a traditional personal pension or stakeholder pension, then a SIPP is likely to offer you more investment options and greater control over how you manage your pension, but will probably have higher fees too. Or if you have a defined benefit or final salary scheme through your work, these tend to offer less flexibility in comparison (but can still prove highly valuable nonetheless).
However, if your employer is offering a workplace SIPP, which some companies do, then there is likely to be very little difference to a SIPP that you arrange yourself. It’s possible that fees will be a little cheaper with a workplace SIPP if your employer has negotiated a discount with the SIPP provider in return for selecting the scheme for its employees.
Employers will pay into a workplace SIPP that they offer in the usual way, but won’t usually pay into any personal SIPP that you might have.