Purely speaking of dragon vs tower. The dragon is a better option.
- The dragon gives more gold. (750 vs 975).
- What you lose from the tower will be won back once you killed the enemy tower.
- The enemy team wont be killing the dragon over the next 6 minutes, you will not have to defend that dragon.
- You don't need to defend that tower either.
That said, dragon is not the top lane responsibility.
It's strongly situational. If you can save both, just save the tower first and then do dragon. If the enemy or minions are killing bot or mid tower, it's easy to defend it. You probably already have 3 people at the dragon, and now you simply gank bot or mid before you go back to dragon. Or dispatch 1 and do the dragon with just 2 people.
If we're speaking of the first 3 towers, those will fall eventually anyway, and are not worth that much post laning phase. If we're talking inhibitor towers or base tower, you MUST defend those before doing dragon.
At the very least, make sure you keep the dragon warded and keep the enemy team away from it. That is a good job for the jungler and the support. If i had to choose between killing a enemy tower, or preventing my enemy from killing the dragon, i would certainly save the dragon.
Getting dragon and keeping tower is of course the best option. The responsibility for killing the dragon lies at the bot laners early game. They should push or win there lane, and then kill dragon. If any jungler is around, the route around level 6 or 7 should be either gank bot->dragon or just call bot->dragon. If it's not possible to do the dragon at this point because of enemy aggression (and map control), keep the dragon warded and prevent the enemy team from taking it instead.
Best Answer
A well played Riven can beat almost anyone in lane, especially melee champs. That being said, to deal with her in lane as Olaf I would start Cloth Armor + 5 pots and then build into Ninja Tabi and Wriggle's Lantern. These items allow you to sustain her harass through both increased armor and lifesteal.
Trading with Riven is very difficult, as she can easily use her abilities to run away or harass depending on how you play. If you notice that she uses several of her abilities in quick succession, take advantage of her 6-7 seconds of cooldowns to try to get your true damage off as she attempts to CS. Otherwise, there is not much you can do to her. Be very careful when she is about to hit level 6, as many Rivens like to go all in right when they get their ult.