ChipFlip
Two $300 cards, three years apart in design philosophy. The Gunnir Arc B580 sits at $299 used, the MSI RX 6700 XT at $299.99. One is Intel's second-generation attempt at the midrange. The other is AMD's proven RDNA2 workhorse from 2021.
VRAM goes to the B580, and it matters more every year
12GB on both, so this one's a draw on capacity. The difference is what surrounds it: the B580 pairs its 12GB with a 192-bit bus and newer memory, and Intel designed the card around modern rendering workloads rather than retrofitting for them.
The 6700 XT's 12GB was generous for 2021 and remains adequate now. Neither card will run out of memory at 1080p or most 1440p settings.
Raw rasterization favors the 6700 XT
In straightforward, no-ray-tracing rendering, AMD's card is generally the faster of the two. RDNA2 was a strong architecture and the 6700 XT was positioned above where the B580 targets. If you play competitive shooters at high frame rates with settings turned down, that's the card that gets you more frames.
Ray tracing and upscaling flip it
Intel's ray tracing hardware is a generation ahead of RDNA2's, and XeSS on Intel hardware runs through the XMX cores rather than the compatibility fallback other vendors get. In titles that lean on either, the B580 closes the gap and often passes the 6700 XT.
There's also the age question. A used 6700 XT has been in someone's machine for up to four years, quite possibly mining for part of that. A B580 is a 2024 card. Warranty and remaining lifespan aren't nothing at these prices.
Bottom line
Buy the B580. Newer silicon, better ray tracing, lower risk on card age, and the rasterization deficit is small enough that it only shows up if you're chasing high frame rates at low settings. Pick the 6700 XT only if that specific use case is yours and you can verify the card wasn't mined on.