RE: SIPP or ISA.20 Nov 2023 19:28
Assuming that you invest in your ISA and SIPP on similar terms i.e. similar costs, similar risk profile, similar portfolio etc. then the general rule of thumb for ISAs vs SIPPs is that you are always better off investing in a SIPP, rather than an ISA, if you expect to be a basic rate taxpayer at retirement, because of the additional compounding effect from the tax rebate, despite the fact that any income you draw from your SIPP might potentially be taxable.
If you are currently a higher rate, or above, taxpayer and expect to be a higher rate, or above, taxpayer when you retire then the calculation is a bit more complex. On the face of it you might appear worse off (the after tax return from, say, a £1,000 being invested in a SIPP being less than £800 being invested in an ISA on similar terms). However, that would be to ignore the upfront, additional tax rebate/reduction that such taxpayers receive from their employee pension contribution (in effect their basic rate band is extended).
Also, it should be born in mind that, on death, a SIPP can now be passed onto your spouse and/or children outside of your estate (exempt from IHT) and retain its tax-free wrapper (income and gains remain tax-free until withdrawn) whereas an ISA can only be passed on to your spouse and retain its tax-free wrapper (on the death of you and your spouse, any shares held within your ISAs, excluding potentially AIM shares, become part of your estate for IHT purposes and any income and gains generated thereafter are potentially taxable on your beneficiaries, should they choose to retain your investments).
That said, it makes sense not to put all of your eggs in one basket. SIPPs and ISAs have different rules that the government can choose to change in the future; so it always makes sense to take advantage of both, where you can, to ensure that any changes have the minimum impact.