I suspect that this is a very nationally biased question, since for example in the UK we have microlight (not quite ultralight, closer to sport pilot) licenses which many countries don't, but no glider licenses.
However, trying to remove the "cultural bias", if you are looking to prepare for military training, I'd say go for gliding where you'll learn situational awareness and good stick-and-rudder skills that are universal and will stand you in good stead. The civil and military approach to procedures is very different in powered aircraft and you will in PPL type flying develop habits that whilst perfectly good in civil flying will be regarded by military instructors as "bad" and can be hard to shake off.
G