I would say Night Vision, no question. Being career military, I feel confident in saying that you will get much more practical day-to-day use out of NVGs than you would thermal.
Thermal does have its place. When I first joined the Army, back in the '80s, I was in Armor. We would use thermal sights, even in daytime, because other armored vehicles stick out like a sore thumb, day or night, when viewed through thermal. Doesn't matter how much vegetation they're hiding behind. However, that's a very specialized situation obviously.
Night Vision (we used to call it Starlight) will give you much more practical use in many situations besides just watching for adversaries.