Make-your-mind-up-time approaches. There are 28 days left to invest your Isa (individual savings account) tax-free allowance.

In the next tax year, investors will be tempted by the Great British Isa, which allows an additional £5,000 in UK shares, on top of the existing £20,000 allowance.

Few details, however, are available at this stage. And for those looking to invest this year’s allowance, there are some difficult decisions regarding the US, which boasts more world-leading companies than any other market. Investors eyeing the US are likely to be torn between the fear of missing out and the fear of buying at the peak. 

There are strong arguments ‘to feel the fear and do it anyway’, as Susan Jeffers, the American self-help author advises. Gerrit Smit, manager of the Stonehage Fleming Global Best Ideas Equity fund, which owns the US tech giants Alphabet and Microsoft, says: ‘There’s more risk of not being invested in the US, than in being invested.

‘The nation has the world’s best entrepreneurs, and an economy with the best chance of staying resilient in current conditions.’

As Jason Hollands of Bestinvest points out, US markets tend to prosper in presidential election years: ‘In the 19 elections since the end of the Second World War, the S&P 500 has delivered a positive return on all but two occasions – which were in 2000 when the dot-com bubble burst and 2008 at the height of the global financial crisis.’

But there is still nervousness, following a 28 per cent rise in the S&P 500 index over the past 12 months to 5,130.95. This is largely thanks to excitement about the AI revolution which has caused shares in the Magnificent Seven – Mag 7 – tech giants to double or close to treble over the past year.

The ‘Mag 7’ are Alphabet, Amazon, Apple, Meta, Microsoft, Tesla and semiconductor maker Nvidia.

The AI data centre server specialist Super Micro Computer has recently joined the S&P 500 following a 276 per cent jump in its shares since the beginning of January.

This week Bank of America raised its forecast for the S&P to 5,400. But the Shiller Cape ratio – a guide to future returns – is sending out danger signals.

Against this backdrop, US investors are either Team Kolanovic or Team Kostin. Marko Kolanovic, the JP Morgan Chase & Co guru, warns that a bubble is forming. David Kostin, his opposite number at rival investment bank Goldman Sachs, argues that even the valuations of tech stocks, ‘are supported by the fundamentals’. If you side with Kostin and can afford to take a longer-term view, you should still be braced for bouts of nervousness.

Donald Trump, if he is elected president in November, could seek to limit Big Tech’s influence. Trade relations could become more troubled, with tariffs on imports, including those from China, which would be charged at 60 per cent.

Hollands says: ‘It’s difficult to know how these factors might play out for stock markets, but it clearly injects a degree of uncertainty.

‘The S&P 500 is trading at prices which are 21 times projected earnings for the next 12 months – that’s 33 per cent above the 20-year median level of 15.8 times.

‘On a price-earnings ratio basis, S&P 500 companies are trading at twice the ratio of those in the UK.

‘Buying US shares means you’re paying twice as much for their profits than if you bought UK shares.’

The Bloomberg Magnificent Seven Total Return Index shows these stocks are trading 38 times their earnings, suggesting that a pullback in prices may be likely. Yet, as Smit argues, the number of people waiting for weakness could soon propel prices skywards again. In light of this, you need to develop a strategy for your American Isa adventure.

There is no obligation to commit the full £20,000 to this market. You could invest a chunk, or start a monthly contribution plan into a fund or trust. This is an excellent route for younger investors.

Household name US and global investment trusts such as Alliance, F&C and JP Morgan American have substantial Mag 7 stakes.

Allianz Technology (where I am a holder) and Polar Capital Technology focus on Big Tech.

But if you already own these trusts, diversification makes sense, given that, if interest rates are cut, the rally could extend from tech to other industries.

The beneficiaries could include the ‘affluent boomer renaissance’ stocks. The view is that this demographic is set to spend big on holidays and healthcare, including weight loss drugs.

It has lifted shares in cruise operator Carnival and pharmaceuticals group Eli Lilly.

David Harrison, manager of the Rathbone Greenbank Global Sustainability Fund, says that some healthcare stocks have been hit by concerns that weight loss drugs could reduce the need for other treatments. He adds: ‘These falls have been overdone.’

Funds that provide a broad spread of US exposure include Premier Miton US Opportunities and the Invesco FTSE RAFI US 1000 ETF (exchange traded fund).

Take a deep breath and trust that these businesses born in the USA may prove to be exceptional.