I play Talon a lot in ranked, but I still say Riven. Here is what's what.
Madreds Razor/Wriggles. Talon, and Riven both do well with this item. Talon is better off waiting for a thirster though.
Armor Pen items. Talon scales much better with these. Outside of Yomuu's Ghostblade I pretty much laugh at any armor pen item on Riven.
Trinity Force. I don't like this item on either champ but Talon would win at picking up that item.
Before I go into a comparison, the one for Talon is meant to be taken lightly, don't flame me.
Talon is like a physical version of Akali. Not really hard, packs a punch, scales well with lifesteal, and has minor CC and decent utility.
Riven can't really be compared to anything, except maybe gragas. Riven has multiple uses for most of her skills. However, unlike Talon riven can comeback from a losing game a lot easier through sheer cc, and chasing/escaping power. An underfarmed Talon is easy enough to keep underfarmed. An underfarmed Riven still smacks people around.
And, uhh sorry to anyone who said otherwise, but Riven has just as low of a skill cap as the rest of the melee champs..