Any character can win a 1v1, 1v2, 1v3. It depends on how smart/coordinated/built/positioned the enemy are.
Riven I absolutely LOVE for late game(her early/mid game are also very well too). Her ultimate is such a massive damage increase plus a ranged execution with extra range for abilities/auto-attacks. Damage Riven with a guardian angel is just lovely.
Darius is different. His early game/mid game are his key points. He can bully people out of lane and execute them properly at 6. His passive is like GangPlanks but gives him a better advantage when he uses his ultimate. Late game Darius, in my opinion, is just sit there and take damage while executing the enemy if they are low. Tanky Darius is recommended(from me).
In my standpoint, I adore Riven. So I would buy her(even though I own here, but for your sake)