I assume you mean a tension and compression diagonal or a pair of tension only diagonals.  Both solutions have applications.  I vote for the one which makes the most sense for a given case.

I either use one "X" brace or two diagonal braces opposed to each other.  Whatever fits best.

I found this PDF at some point that may prove to be useful
http://www.modernsteel.com%2FUploads%2FIssues%2FMay_2006%2F30753_shneur_web.pdf
 
Look at item number 7.  
I tend to prefer chevron braces since this will cut down on the unbraced length of the braces.

I'd say it depends largely on the architectural layout.  Typically, if I have the freedom I prefer the two-story x-brace as it doesn't require as many bays as single diagonals would and eliminates the issue with unbalanced forces that happen with chevron or v-bracing.

A single diagonal brace has it's merits, but also it's limitations.  In high seismic areas you need to have tension diagonals balancing your compression diagonals or else you have to design the brace for the overstrength factors.  Once you design the brace for the overstrength factor this essentially eliminates your redundancy in the design as your collectors, etc. will no longer have amplified forces as compared to your brace.

I also wanted to add that it's worthwhile considering BRBFs (Buckling Restrained Brace Frames) if you are in a high-seismic area.  You'll get a higher R-value, therefore your foundation design will go down.  You also, don't have to design for the unbalanced forces that you have to with a SCBF so therefore a chevron configuration becomes more desirable.  From my understanding there hysteresis loop is very stable and should therefore provide performance that far exceeds the typical brace frame.