There isn't a simple answer to this but it would depend on your household gross income to a large degree as to which way you would choose to go.

You can contact the HMRC helpline on 0845 300 3900 for help or go to the HMRC website and use the childcare indicator.

The rule of thumb we use is that if your household income is below approx £15000 then put your childcare costs through your Tax Credits to the maximum and anything above that claim vouchers. If your gross household income is above £15000 then claim vouchers first and put the excess through tax credits.

It is not an exact science unfortunately so you must always seek advice. It is unusual to be disadvantaged by taking vouchers and sometimes it can have no effect