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.
Minions gain health over time, so if you barely 1-shot them now you won't 1-shot them in the next cycle which is nicely explained on this page: http://www.lolpro.com/guides/game-play/166-game-play-guide-last-hitting
Minions have an "Upgrade Cycle" that activates every 3 minutes. Despite spawning only at 1:30 the cycle already begins at 0:00. Meaning that after 3 minion waves a minion's power and gold worth is already higher than the first 3 waves.
Melee
Health: 445 (+20 per cycle)
Armor: 0 (+2 per cycle)
Magic Resistance: 0 (+1.25 per cycle)
Caster
Health: 280 (+15 per cycle)
Armor: 0 (+1.25 per cycle)
Magic Resistance: 0 (+2 per cycle)
Siege
Health: 700 (+27 per cycle)
Armor: 15 (+3 per cycle)
Magic Resistance: 0 (+3 per cycle)
Super
Health: 1500 (+200 per cycle)
Armor: 30
Magic Resistance: -30
Ahris damage is only influenced by level/AP and wether you hit opponents in both directions.
Best Answer
Because killing a tower doesn't prevent the other side from getting the same pot of gold, whereas killing dragon does. A ward at dragon is all well and good but the only way to be sure the other team doesn't get it is by killing it.
It's the same reason m5 have done so well in Kiev and Hannover, they counterjungle relentlessly because it's gold you can earn that also prevents the other team from getting it. Basically, in the big race to be the richest team, gold from neutral sources counts double!