Depends on what you are doing - sometimes, it can be solved by simple arithmetic, which is faster than both. What exactly are you trying to do?

For example, if a number is 5, make it 2, if it is 2, make it 5. It could be coded as

Technically, a Switch / Case statement is faster because in an IF Statment, each IF / ElseIF is evaluated. In a Switch/Case, once the condition is met, the Switch/Case is exited.
Personally, I find Switch/Case to look cleaner and more readable as well. If it is just a small check as in xwb's example, then I would just use a standard IF or to shorten it I would use an InLine IF. The syntax differs between VB and C#

It really depends on the internal implementation of the switch statement.

If the cases are sequential, very often, the implementation is the equivalent of a computed goto. For example
The compiler would arrange them as 5, 6, 7,8, subtract 5 from x and use a computed goto.

If the cases are not sequential, they could be sorted into a lookup table followed by a computed goto. Alternatively, it could be implemented as an if-elseif-else statement. If it is the latter then there is no difference between the switch and the if-elseif-else other than possibly elegance of the code.

An IF statement will always evaluate each condition. A switch will fall through once the condition is met. True, depending on the conditions, they could be equal in performance, but the switch will win out in most cases.