终于vs 到底

[MaLaTang]

终于她买了新自行车。。。

到底她买了新自行车

are daodi and zhongyu interchangeable in all cases?

[lukoktonos]

I'm not sure about all the possible uses, but one use that 到底 has which 终于 does not is to add emphasis to a question.

你到底去哪里？Where in the world are you going? (Where in the heck do you think you're going? kind of feeling)

[shinewind]

My conclusion is that 终于is simpler while 到底is a little bit complicated with at least 3 levels of meaning:

终于=finally/at last… example:他终于还是来了(finally he came)。

到底:

• Finally/at last… example:他到底还是来了finally he came)。
  
• On earth. example:你到底想干吗？（what on earth do you want to do?）
  
• After all… example: 她到底是你的女儿 (she is your daughterafter all.)