I suspect nights/stays will be combined for 2019 status, but that means that it doesn't really matter how you approach it, factoring in only next year.
But for 2018, focusing on one program will get you there faster. 50 nights at SPG will get PLT, then you can match to MR. 25 in each won't get you PLT in either.
If your value equation between the two is, for some reason, substantially different than the 3:1 already announced, that could affect your decision.
IMHO, I'd focus on one program and, all else being equal, it would be SPG with the lower night requirements.
Already having LT or current PLT status in either program would certainly influence this decision, as would the number of nights you expect to stay this year.