I would generally go for Title Case, or even Sentence case. It is widely accepted that using lower case letters makes text easier to read, because the letter forms are more distinct. Personally, I don't like overuse of capital letters (I'm forever taking them out of my bosses' work) and so only use them where they are absolutely required. On the other hand, if you're only going to have short extracts of text, you may consider ALL CAPS to be a suitable stylistic choice - but if you do, I would suggest having Sentence case in the code, and using CSS (text-transform) to capitalise it, and also to increase the letter spacing slightly, as this will off-set the shoutiness and make it a bit easier to read.
Ideally you should have enough visual clues (colour, border, spacing, positioning) that an item is more important that you don't need to rely on casing to convey that.