If you're looking to add an S&P/ASX 200 Index (ASX: XJO) private health insurer to your investment portfolio, should you buy NIB Holdings Ltd (ASX: NHF) shares or Medibank Private Ltd (ASX: MPL) shares?

It's a question the analysts at Macquarie Group Ltd (ASX: MQG) tackled in a new research report, released on Friday.

First, taking a look in the rear view, NIB shares are up 26% in 2025 and currently trade for $6.98 apiece. The Australian health insurer's shares also trade on a 3.9% fully franked trailing dividend yield.

Medibank shares have enjoyed an even stronger run so far in 2025. Currently trading at $4.93, shares are up 30% year to date. Medibank shares also trade on a 3.5% fully franked trailing dividend yield.

Of course, those gains and dividends have come and gone now.

So, here's what Macquarie expects next for the ASX 200 health insurers.

Will NIB shares outperform Medibank shares?

Following on the strong run higher in 2025, Macquarie believes both Medibank and NIB shares could come under pressure in the year ahead.

On the valuation front, the broker noted:

MPL is trading at a ~36.9% premium vs NHF (vs a ~3.3% long-term premium). Despite that, MPL's 1-year forward PE is ~100bps below its own 5-year average (excl. the period of concern relating to COVID-19 and the 2% price caps) making valuation signals inconsistent at this point.

Macquarie's analysts added that private health insurers "should serve as a relatively safe haven in a tougher economic environment, particularly as claims volumes remain lower in the short term. Having said that, at current levels valuations do not appear attractive".

The broker maintained its neutral recommendation on Medibank shares with a 12-month price target of $4.25 a share. That's almost 14% below current levels.

Macquarie is even more bearish on NIB shares. The broker maintained is underperform rating with a 12-month price target of $5.55 a share. That's more than 20% below current levels.

What's the latest from the ASX 200 health insurers?

Both NIB shares and Medibank shares posted some big gains in February following the release of their earnings results.

Medibank reported its half-year results on 27 February.

Highlights for the six months to 31 December included a 6.1% year-on-year increase in revenue to $4.27 billion. And underlying net profit after tax was up 13.8% to $299 million.

The Medibank share price closed up 10.0% on the day the company reported.

NIB reported its own half-year results on 24 February.

Highlights from those results included a 7.7% year-on-year increase in revenue to $1.8 billion. Net profit after tax (NPAT) went the other way, falling 20.2% to $83 million.

Amid record first-half sales growth, NIB shares closed up 12.5% on the day the Aussie health insurer reported.