In depends on how you're using the text.
You can increase and decrease visibility of text by applying different styles to your text. Let's focus on things you can do to increase visibility of text, since that's what your question is about.
When to use all bold or WHEN TO USE ALL CAPS
Use ALL CAPS or bold to put emphasis on text you want to get your users' attention to.
You can increase text visibility for text you want your users to see first by making it either stand out in size (UPPERCASE) or visual weight (Bold). Just look at how the two examples from the previous sentence stand out.
Let's take the examples you give in your question;
Order today and SAVE BIG
'SAVE BIG' stands out. You can use that to put attention on 'SAVE BIG'. I can imagine that's what you want when you design something for retail/sales. Only use all caps for small strings you want to put emphasis on.
There's a great article about text visibility here. Check out number 5.
When not to use all caps
Do not make long body text all caps or bold.
At body text sizes, capital letters—or simply caps—are harder to
read than normal lowercase text. Why? We read more lowercase
text, so as a matter of habit, lowercase is more familiar and
thus more legible. Furthermore, cognitive research has
suggested that the shapes of lowercase letters—some tall (dhkl),
some short (aens), some descending (gypq)—create a varied visual
contour that helps our brain recognize words. Capitalization
homogenizes these shapes, leaving a rectangular contour.
Source: All caps
Also do not make labels bold or all caps.
However, bold labels resulted in an almost sixty-percent increase in
saccade time to move from the label to the input field—from 50ms
without bold labels to 80ms with bold labels—with no apparent
advantage from the more prominent labeling. Bold labels were more
difficult for users to read and perceive—probably because there was
more visual confusion between the bold text and the heavy adjacent
borders of the input fields.
Source: Label Placement in Forms
Don't forget to look at your corporate identity/style. Is a bold font or all caps part of your branding?