Buy the Seiko if you want a versatile watch with real mechanical movements and dressier looks. Buy the Casio if you want a tough, feature-packed watch that survives anything and costs less. Both are excellent. The right pick depends on how you'll use it.
The short answer
Seiko builds watches that feel like a step up in polish and craftsmanship, including automatic movements you never have to battery-change. Casio builds watches that shrug off abuse, run for years on a single battery, and pack in functions like alarms, timers, and world time. Pick Seiko for style and mechanical charm, Casio for toughness and utility.
Movements and accuracy
Casio is almost entirely quartz, which means excellent accuracy and near-zero maintenance. Seiko offers both quartz and its own in-house automatic movements. A quartz watch keeps better time day to day, but a Seiko automatic gives you the sweeping seconds hand and self-winding mechanism many enthusiasts love.
Durability
Casio, especially the G-Shock line, is the benchmark for durability. Shock resistance, big water resistance ratings, and resin cases make them nearly indestructible. Seiko watches are well built too, but a metal-cased dress or dive Seiko is not designed to be thrown around like a G-Shock.
Style
Seiko wins on versatility. You can find a Seiko that works with a suit, a Seiko diver for the weekend, and a rugged field Seiko for daily wear. Casio leans sporty, digital, and casual, though its slim vintage-style pieces are quietly stylish and cheap.
Price
Casio is the value king. You can buy a genuinely good Casio for well under $50. Seiko starts higher, and its automatics climb into the low hundreds, but you get more finishing and a mechanical movement for the money.
Which should you choose?
Choose Seiko if you want one versatile watch that looks sharp and introduces you to mechanical movements. Choose Casio if you want maximum toughness, functions, and value, or a beater you never have to worry about. Many people own both, and that's a smart move.