What is the difference in memory allocation and performance by using

StringBuilder.Append("Str1");
StringBuilder.Append("Str2");

and 

StringBuilder.Append("Str1"+"Str2");

Ok, I wrote a little test application to time this because you got me interested.

Here's the calculation I used (based off your example).

// Test single appends, going 10,000,000 times.
 for (int i = 0; i < 10000000; i++)
          _SB1.Append("Test");
// Test append + append going 5,000,000 times.
for (int i = 0; i < 10000000/2; i++)
          _SB2.Append("Test" + "Test");

The results (in milliseconds):

First Case (single appends)
77ms
71ms
78ms
73ms

Second Case (Append w/ 2 String +'s)
48ms
61ms
57ms
57ms

So, going half the amount of times, doing the same amount of text, adding strings within an append call is quicker than doing multiple single appends.