Measuring time is about more than merely knowing when events occur. It's a vital prerequisite for synchronizing your operations, securing your data and serving your clientele. It pays to be precise, accurate and on-time, and your ability to do so depends on how good your time server is.
Unfortunately, you're unsure which kind of time server is best. You've narrowed it down to two choices: internet-based NTP and GPS-referenced Stratum 1 NTP. The distinctions aren't so clear, but these facts might make it easier to pick wisely.