Exor S.p.A.'s $6.4 billion all-cash offer to acquire Bermuda reinsurer PartnerRe Ltd. introduced a new player into the already bustling reinsurance merger and acquisition space that presents a financial alternative to the strategic merger proposed by Axis Capital Holdings Ltd.

The April 14 offer is a direct challenge to the $11 billion all-stock merger proposed in January by Axis Capital Holdings Ltd., giving PartnerRe shareholders two very different offers to consider while putting Axis on the spot.

In an April 15 investor call, Exor Chairman and CEO John Elkann said “the reinsurance sector continues to earn attractive returns,” acquiring PartnerRe would diversify Exor's portfolio and offer the “opportunity to exploit synergies between Exor investment activities and the PartnerRe investment portfolio.”

In a statement, PartnerRe, which had already signed an amalgamation agreement with Axis, said only that it will review Exor's proposal.

“I think it puts the Partner shareholders in a fortunate position because now they have two offers,” said Cliff Gallant, an analyst at Nomura Securities International Inc. in San Francisco, adding that Exor is not one of the typical players in reinsurance M&As.

Italy's Exor S.p.A. is a European-listed investment company with a net asset value of more than e10 billion ($10.81 billion), according to its website. It holds a roughly 29% stake in Fiat Chrysler Automobiles .

Analysts see value in both offers.

“Numerically, it's a superior offer,” Amit Kumar, New York-based vice president and senior analyst of insurance at Macquarie Capital (USA) Inc., said of Exor's bid.

“With the other offer (by Axis), you are buying a future value promise, which may or may not be realized based on what happens or does not happen in the market,” said Mr. Kumar, who in analyst notes recommended PartnerRe shareholders accept the Exor proposal.

“I'm sure the Partner shareholders are thinking about near-term returns and what the benefit is to them, and it's hard to turn down cash that's certain and a little bit more of an immediate payoff than what Axis is offering,” Mr. Gallant said.

Exor “explained themselves pretty well in their (investor) call in terms of what the return hurdles are. Partner seems like a company with a good balance sheet, and it produces good cash flow, so it kind of makes sense for Exor. It seems like a pretty legitimate offer,” he said.

Still, Mr. Gallant said, “I do think the Axis-Partner combination seemed compelling; it seems like there are a lot of positives to that combination.”

“Certainly (Axis) management believes, and I also believe, there is compelling rationale in combining these insurance enterprises,” said Meyer Shields, managing director at Keefe, Bruyette & Woods Inc. in Baltimore. “I think as long as they cover most of the gap between what Exor has offered, probably in the form of a special dividend, then I think they can make their pitch to shareholders.”

In an internal memo to Axis employees, Axis CEO Albert Benchimol said, “We are fully committed to our combination with PartnerRe.”

“For now, nothing changes,” Mr. Benchimol said in the memo. “We will continue to execute on Axis' business plans for 2015 to meet the expectations of our existing shareholders while moving forward on integration planning underway with PartnerRe.”

The already competing bids for PartnerRe could have some company.

“We assume that there could be some potential room for additional bids coming out there,” said Brian Schneider, Chicago-based senior director of insurance at Fitch Ratings Inc., which placed PartnerRe on rating watch negative following announcement of the agreement to merge with Axis.

Following Exor's bid, Standard & Poor's Ratings Services revised its outlook of Exor to negative from stable while affirming its BBB+/A-2 corporate credit rating.

Questions of valuation also have arisen, with analysts saying PartnerRe's redundant reserves may make it potentially more valuable than at first glance.

“I think it's widely believed that Partner has excess reserves, and so its book value might be understated,” said Mr. Gallant. “So, you could pay above stated book and still be getting a good value.”

Saying PartnerRe is “considerably over-reserved” with a strong record of reserve releases, Mr. Shields said PartnerRe's “economic value is higher than the current share price,” which was $128.96 at the market close on Friday.

Given these facts, “it wouldn't be difficult at all to justify a price in the $135 per share to $140 per share range,” Mr. Shields said.

Should the Axis-PartnerRe merger not be completed, Axis would receive a breakup fee of $250 million, “equal to approximately 30% of (Axis) 2014 earnings,” Moody's Investors Service Inc. said in an investor note that described Exor's bid as “credit negative” for PartnerRe and Axis.