Both a Cash ISA and a Stocks and Shares ISA shelter your money from tax, and both sit inside the same £20,000 annual allowance for the current tax year. The wrapper is identical. What goes inside it is not. Choosing between them comes down to three questions: when will you need the money, how would you feel if the balance dropped 20 per cent for a year or two, and what are you actually saving for?

How a Cash ISA works

Think of it as an ordinary savings account with a tax-free badge. You deposit money, the bank or building society pays interest, and you owe nothing on that interest. Rates move around, so you will see easy-access accounts, notice accounts and fixed-rate deals typically ranging from around 1 per cent to over 4 per cent depending on the market and how long you lock your money away.

Cash ISAs are covered by the Financial Services Compensation Scheme, which protects up to £85,000 per eligible person, per financial institution. You can open one from age 16, and the money cannot fall in value unless the institution fails. That certainty is the whole point.

One thing worth knowing: the Personal Savings Allowance already lets basic rate taxpayers earn £1,000 of interest a year tax-free outside an ISA, and higher rate taxpayers £500. For smaller balances, a Cash ISA is less about tax today and more about protecting yourself if the allowance is reduced later. It is also simpler to keep track of.

How a Stocks and Shares ISA works

Here your money buys investments — usually funds or exchange-traded funds, sometimes individual shares, investment trusts or bonds. You pay no tax on dividends and no capital gains tax on growth inside the wrapper, which matters once you are dealing with meaningful sums.

The trade-off is volatility. Markets fall as well as rise, sometimes sharply and for extended periods. A diversified global fund might be down 15 per cent one year and up 25 per cent the next. That is normal, not a broken plan, but it only works if you can leave the money alone long enough to recover.

Costs matter more than most people expect. You will typically pay a platform charge of roughly 0.2 to 0.5 per cent a year, plus an ongoing charge on the funds themselves, often 0.1 to 0.7 per cent. On a £30,000 pot, a difference of half a percentage point is £150 a year — real money over a couple of decades. Investments are not covered by the FSCS in the same way cash is, because the risk of loss is yours by design.

Timeline matters more than anything else

This is the single most useful rule of thumb. Money you will need within five years belongs in cash. Money you will not touch for ten years or more is usually better invested, because history suggests diversified markets tend to grow over long periods — though nothing is guaranteed, and past performance is not a reliable guide to the future.

Between five and ten years, it depends. If you are saving for a house deposit and a two-year delay would wreck your plans, keep it in cash. If you are saving for retirement in your forties or fifties, holding everything in cash for decades risks inflation quietly eroding the value of your pot, even if the headline number looks safe.

Risk, access and the details people forget

• Build a cash buffer first. Three to six months of essential spending in an easy-access account, ideally an ISA, before you invest anything.
• You can hold both. Splitting your £20,000 allowance between a Cash ISA and a Stocks and Shares ISA is perfectly normal and often sensible.
• You are no longer limited to one ISA of each type. Since April 2024 you can pay into more than one Cash ISA or Stocks and Shares ISA in the same tax year, as long as your total stays within the £20,000 limit.
• Transfer properly. To move money between ISAs, use the formal transfer process through your provider. Withdrawing and redepositing can eat into your allowance unless the account is a flexible ISA.
• Flexible ISAs let you replace money. If you take cash out, you can put it back without using up fresh allowance — but not every provider offers this, so check.
• Cash still carries risk. Over long periods, inflation can outpace savings rates, leaving you with less real buying power.

How to decide in practice

Work through it in order. Clear expensive debt first, since no ISA will beat 20 per cent interest on a credit card. Then build your cash buffer. Then set aside money for short-term goals — a car, a wedding, a deposit — in a Cash ISA. Only then start investing for the long term, and do it monthly rather than in one lump if that helps you sleep at night.

You do not need to pick shares or time the market. A low-cost, globally diversified fund held inside a Stocks and Shares ISA is a perfectly respectable approach for most households, and it is easy to automate. Review your split once a year, or whenever your circumstances change. If you are unsure, holding more in cash while you build confidence is not a failure — it is a sensible starting point.