guidelines - Should we use 'Title Case' or 'Sentence case' for headlines and buttons?

I am wondering if we should use Title Case or Sentence case for buttons and headings in websites and web/mobile apps?

Title Case:

   

Sentence Case:

   

In Yahoo's UI guidelines I couldn't find any specific recommendation except for 'choose your style and be consistent'.

Is that all? Or are there any other (additional) recommendations. Like for specific use cases or apps where the one style would be preferred over the other?

Edit: The question refers primarily to English, but differences in other languages are also good to know.

Answer

I prefer 'Sentence case' over 'Title Case' because sentence case respects the difference between proper nouns and the other words.
I always thought that it was customary in English.
In Spanish it is not, we use sentence case, like this traditional argentine newspaper does.
This traditional USA newspaper uses Title Case instead.
These are language differences. For the Spanish for Spanish is not capitalized, same as the weekdays and month names.
US English is more capitalization-prone than Spanish.
UK newspapers use sentence case.
I suspect that Title Case propagates in Spanish pages because it's easier to write and bacause of the influence of the USA sites.