Short verdict
Buy in Korea. Best purchased during a Korea trip, usually because of local access, price, testing, or limited bundles.
A T-money card is useful for subway, bus, taxi, and convenience-store payments.
What to know
Buy after arrival if you are comfortable using airport convenience stores or subway stations. Buy a bundle online only if it includes pickup, SIM, or airport transfer value you actually need.
What to avoid
Skip overpriced souvenir bundles that add little beyond the card itself.
Final verdict: buy in Korea for most travelers.