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.
I generally assume people know which lane has which champion so I only say: mid mia
There are cases in which I do other things:
I know which way enemy champ is going - I use several pings to highlight champions path with an additional back ping if my teammates don't respond.
My opponent has been gone away - I say "mid still mia, prob [whichever lane is most exposed]"
Enemy jungler walks through my lane - "[jungler] going [direction]"
Best Answer
This strongly depends on the match-up and how well you're doing. If you're against someone who you can trade against easily and always come out on top in said trades getting a Dorans will generally be a better idea because it'll improve your damage more than low level alternatives. This is generally true if you're playing a champion that hits harder versus a more tanky champion that doesn't deal as much damage. Say Riven versus Cho'gath for an example, Riven would probably want to grab x2 Dorans before going into her build to give her more damage potential on Cho'gath since she normally comes out on the good side of trades.
Alternatively, if you're getting bullied pretty badly you can always go for Dorans items (they're fairly gold efficient), though I'd probably recommend getting a Wriggles because of higher sustain and the damage reduction. A lot of top lane is knowing your preferred champion's match-ups and how well he/she trades against their enemy in lane.
For a little bit a math on Dorans Vs Wriggles I'm gonna add a link to another answer on here. Just for Context: The question is about Dorans Vs Wriggles on an AD carry, but the math still applies universally. https://gaming.stackexchange.com/a/59537/7494
Additionally, if you get fed early and can afford beefier items you should almost always go for those, but getting Tier 2 Boots, x2 Dorans, and a cheap defense item at low levels will almost always be better than trying to save for a 1.5k+ gold item early.
Hope this helps :)