Liquidity vs transparency
USDT has the deepest global and emerging-market liquidity; USDC is favoured for its reserves transparency and regulatory posture. Most cards support both.
Chain support
Check which chains each card supports for each coin — it affects top-up fees and speed more than the coin choice itself.
Practical takeaway
In emerging markets, USDT often wins on local liquidity; in the EU/US, USDC is frequently the default. Either works on the leading cards.