There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:
// File1.cs
using System;
namespace Outer.Inner
{
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now imagine that someone adds another file (File2.cs) to the project that looks like this:
// File2.cs
namespace Outer
{
class Math
{
}
}
The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.
This changes if you put the using inside your namespace declaration, as follows:
// File1b.cs
namespace Outer.Inner
{
using System;
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now the compiler searches System before searching Outer, finds System.Math, and all is well.
Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.
It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.
Use whatever you and your team find the most readable.
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.
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.
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.
Best Answer
It depends on where your priorities lie. If you're interested in furthering yourself as a developer, TDD is definitely worth looking into if only for the experience. It'll help you rethink the way you code and probably make you a better developer because of it.
But that could easily be outweighed by how much TDD hampers your ability to get your product out in a timely manner. You mentioned that you're the only developer working at this company and that means that the pressure is on you to get your project done. TDD is certainly a great practice, but sometimes real life constraints and practicality must come first.
So in short, if you can spare the time then yes, use TDD. It's actually not that much overhead and you can always skip the testing in a pinch. But if you're really strapped for time and don't think you'll be able to incorporate it without putting your product and job at risk, nobody would fault you for skipping it. The purists will disagree, but it's not a black and white world and sometimes compromises must be made to get things done.