Just the metals line item
Massive.com (formerly Polygon.io) and Twelve Data both cover gold and silver as a small slice of a stocks-first or multi-asset-first catalog. This comparison stays inside that slice, futures settlement, spot pricing, and streaming for metals specifically, and doesn't try to referee their much larger equities and options rivalry.
Pricing model
Twelve Data offers $79/month Grow at 377 credits/min ($66 billed yearly). Entry Grow is $29/month for personal use at 55 credits/min. Pro and Ultra cost more and raise the credit allowance; see its pricing page. Credits are weighted by endpoint.
Futures settlement, the real difference
Massive.com's own documentation confirms gold, silver, and copper futures (GC, SI, HG) on its futures plan, the clearest settlement-symbol claim in this whole set of comparisons. Twelve Data's /commodities endpoint lists XAU/USD and XAG/USD spot without a documented futures settlement symbol at that endpoint level. If your product resolves against a settlement price rather than spot, that gap matters more than anything in the pricing tables above.
Streaming
Both platforms ship WebSocket streaming for live prices. Massive.com bundles it into its single $49/mo tier at no extra metering; Twelve Data runs a separate WS-credit pool alongside its regular API credits, starting with a trial pool on Free and Grow and scaling to 10,000 WS credits/min on Ultra.