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.