I think you'll find the immediate vicinities of these hotels to be safe, albeit in corporate office-park surroundings without much to do. Coming from the north, Greenbelt isn't a bad choice for accessing the city, and (taking a Green Line train) you'll be able to get to the ballpark without having to change trains.
You might also want to look at hotels in downtown Silver Spring (Crowne Plaza, Courtyard, Hampton Inn/Homewood or Hilton). Those too are fairly close to I-95, are walkable to the Metro and to many more amenities than you'll find in Greenbelt.
One thing to consider is scheduled weekend track work on the Metro, which now happens fairly often and can cause significant delays (and the need to change to buses to get through closed sections) on the affected line(s). This is something Metro usually posts a few weeks in advance on their website.
I'd consider booking cancelable reservations at one of the Greenbelt hotels you've suggested PLUS one of the Silver Spring hotels. Then watch for pricing changes and the Metro maintenance schedule, and cancel one of them.
Also, prices in DC are subject to change--keep your eyes open there as well, and look for bargains at sites like Travelzoo.