I've just done a very quick & dirty calc (which I'd be grateful if someone would check independently!).
Based on 20 years, 5% growth a year, assuming no uplift in savings rate or tax allowance - obviously both unlikely but don't think it affects the direction of the outcome.
ISA - 10k of your salary = 8k to invest (ignoring NI)
Pension - 10k of your salary = 8k to invest, plus tax rebate (claimed by SIPP provider) = total investment of 10k per annum.
ISA - end of 20 years pot has grown to 277.8k. Withdraw at 4% = 11.1k. Minimal tax on 0.5k so 11k after tax.
Pension - end of 20 years pot has grown 347.2k. Withdraw at 4% = 13.9k. Tax allowance of 10.6k, taxable amount 3.3k, tax payable @20% £0.7k = 13.2k after tax
Difference = 2.2k.
Also, you can take 25% of your pension tax free and then withdraw your full tax allowance tax free. That gives you a tax free sum of 86.8k, a remaining pot of 260.4k and withdraw at 4% = 10.4k tax free.
Like I said, quick and dirty. But on the very rough sums above, you can end up with good lump sum of 86.8k and only forfeit 11.0k-10.4k = 0.6k per annum. The drawback being it can't be accessed to 55 of course.