<t>Use whatever you and your team find the most readable.<br/>
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Other answers have suggested that a new string is created every time you use "". This is not true - due to string interning, it will be created either once per assembly or once per AppDomain (or possibly once for the whole process - not sure on that front). This difference is negligible - massively, massively insignificant.<br/>
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Which you find more readable is a different matter, however. It's subjective and will vary from person to person - so I suggest you find out what most people on your team like, and all go with that for consistency. Personally I find "" easier to read.<br/>
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The argument that "" and " " are easily mistaken for each other doesn't really wash with me. Unless you're using a proportional font (and I haven't worked with any developers who do) it's pretty easy to tell the difference.</t>