Title Case or Sentence case for dropdown options in web form?

Question

We're adding dropdown boxes (HTML <select>) to several forms to allow users to choose between several options. Should dropdown options be Title Case or Sentence case?

In our web application we use Title Case for all HTML form labels, headings, and buttons. We use Sentence case for help text and tooltips. Whether or not these are the "ideal" casing standards for these kinds of UI, it would be much too expensive to change now. But when we add new varieties of text to our forms, we need to decide which flavor of casing to use. If it matters, native English speakers are 95%+ of our users, although that's hopefully going to decline to about 80%-90% in the next 5 years if our sales team does well in Europe.

For dropdowns, one argument is that the options are really like radio button labels, which would be Title Case elsewhere in our app.

The other argument is that the dropdown already has a label and the options are really explanatory help text, which would be Sentence case elsewhere in our app.

Which do you think is a more compelling argument, and why? Also, does one of them make localization easier?

FWIW, I took a quick look around the web and the first two examples (from Amazon and Apple, see below) both used Title Case for dropdown labels but Sentence case for dropdown options.

Answer 1

I don't have any "official" or user tested information about this, but in the examples you have detailed in your question, the sentence case works well for answer which are based around a response you would generally hear someone speak.

It makes sense that "value" answers would be capitalized.

i.e.

Express Delivery

Economy Delivery

Answer 2

I find this to be helpful.

http://msdn.microsoft.com/en-us/library/bb158706.aspx

What's important is to remain consistent.