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Marriott Rewards is the larger program. Marriott is the acquiring company. And, chances are, Marriott Rewards will be the surviving program and will look much as it does now. I hope. . .
MR points and SPG points are a different currency. It takes far more hotel spending to accrue the same number SPG points as MR points. However, Marriott's hotel redemptions typically require two to three times as many points as Starwood's hotel redemptions for similar hotels. So SPG members can reasonably expect a multiplier to be applied to their point balances before they are transferred over.
Do you know of another merger where the acquired members received a multiplier?
Yes, SPG points were/are accrued more from hotel spend. They are also transferrable 1:1 (and 1:1.25) to airlines until the program changes. Any member can transfer them before the change to maintain this coveted value.
I looked at 2 comparable hotels. For 12/22/2015 in Phoenix the Renaissance Phoenix Downtown 20K points or $139 and the Westin Phoenix Downtown 10K points or $179. Both Cat. 4. So your MR redemption being 2X that of spg's holds true for this comparison. BUT, for the exchange of points to miles the spg points have high value 1:1 for many airlines. MR transfer to Aeromexico, Aeroplan (Air Canada), Alaska, American Airlines, British Airways, Copa Airlines, Delta Air Lines, Frontier Airlines, GOL/Varig, Hawaiian Airlines, Iberia Airlines, Virgin Atlantic can be 2.8X - 5X
REWARD POINTS DETAILS
2,000 Miles or Avios 10,000
5,000 Miles or Avios 20,000
10,000 Miles or Avios 30,000
25,000 Miles or Avios 70,000
50,000 Miles or Avios 140,000
So how would you determine a multiplier? I would hope by the hotel since, after all, it is primarily a hotel rewards program.
It remains to be seen if SPG members with lifetime elite status will have that status transferred over on a Gold-for-Gold and Platinum-for-Platinum basis. In any case, I would expect program-to-date Starwood nights to count toward toward Marriott lifetime status. I think we'll see a lot of newly minted Marriott Rewards lifetime elite members coming out of this merger.