Both are falling-sand physics sandboxes, but they cater to very different playstyles. Here's the breakdown.
Platform: Windows/macOS/Linux (standalone), Web (limited). Price: Free, open-source.
Platform: Browser (Java/HTML5), Android/iOS apps. Price: Free (ad-supported). By Dan-Ball.
Verdict: TPT is a simulation engine; PG is a multiplayer toy. Try both—they're free!