This article shows that pensions are generally better than ISAs for pension provision.
The example shown doesn’t take into account matched employer contributions either. The only flaw I see is it assumes growth is the same for each type of investment but the pension fund I am in don’t have a great selection of funds to choose from and my ISA does outperform my pension. However, for a SIPP which has a wide range of investment choices you can assume it will grow at the same rate as an ISA.