I'd use since WordPress is xhtml and all tags should be lowercase.
The bigger question is why you want hard line breaks. That would help determine how to achieve them without using breaks this way -- typically not a good idea unless you have a good reason to do it.
If you simply don't like the space between paragraphs, edit the style.css file to change the margins (top and bottom or both) for whatever rule that affects the paragraphs, probably in .entry p or something similar.
It's almost always a good idea to use a 0 margin-top and set the spacing for the margin-bottom below paragraphs. Note that in modern browsers, vertical margins collapse in favor of the larger value.
If you still need put it between tags, as in <p> </p>
HTH
Casey