Verdict
Chess.com is the best place to play chess and take broad lessons; Mainline is the best place to fix your own recurring mistakes. They're complementary: Mainline analyzes the games you play on Chess.com.
| Mainline | Chess.com | |
|---|---|---|
| Primary purpose | Fix your specific weaknesses | Play games + general learning |
| Training source | Your real games | Generic lessons & puzzles |
| Weakness analysis | Ranks mistakes by rating-point cost, builds a drill queue | Game Review highlights individual game errors |
| Spaced repetition | Yes, on your weakest patterns | Limited |
| Price | Mainline is free. | Free; paid tiers |
Use Chess.com to play; connect those games to Mainline to stop repeating the mistakes that cost you rating.