The round brilliant has fifty-seven or fifty-eight facets, depending on whether you include the culet. The Portuguese cut has at least one hundred and sixty. The difference is not subtle. It is the difference between a flash and a fountain.
Each facet on a diamond is a mirror. Light enters through the table, bounces off the pavilion facets, and exits back through the crown facets. The more facets you have arranged at correct angles, the more bounce points light passes through before it exits. With 57, you get bright, discrete sparks. With 160, you get a dense, overlapping starburst — what cutters call "a stone that lights itself."
Both are round, both are brilliant — but only one fires across a candlelit table.
What this means practically: the Portuguese cut outperforms the round brilliant in low light conditions. A round brilliant depends on direct, overhead light to fire fully — it was optimised for the lighting conditions of an industrial workshop, which approximates midday daylight. The Portuguese was optimised for the lighting conditions of evening dining: low, warm, directional. Where a round brilliant under candlelight reads as quiet, a Portuguese fires.
The round brilliant wins on light return at the high end. Under ideal conditions, with overhead daylight and a perfect cut, a top-grade round brilliant returns slightly more light than even an excellent Portuguese. But the gap is narrow and the conditions are narrow. In the lighting conditions of real life — restaurants, indoor evening events, the soft late-afternoon light of a north-facing window — the Portuguese consistently performs better.
Price is roughly equivalent for equivalent stones. The Portuguese requires a higher-quality rough crystal because its complex facet structure amplifies any inclusions; this cost is offset by the cut's lower retention rate, which means a smaller finished stone from a given rough size. The economics roughly balance.
Choose the round brilliant if your priority is maximum sparkle in well-lit rooms and you don't mind that your stone looks like every other stone. Choose the Portuguese if you wear jewellery primarily in evening light and you want a stone that few people will have seen before.