You should not worry about this level of detail for performance reasons, unless you've identified a performance issue.
However, wondering to satisfy an inquiring mind is a different matter! :-) The answer is they are identical.
A comparison is usually compiled as an instruction which sets condition flags; this could be a specific comparison instruction or something like an arithmetic instruction which sets condition codes; followed by a conditional jump which tests the condition flags - and a test for "equal" is the same cost as for "not equal", just a different setting of those condition flags.
This also means that statements such as if([some method call]) ...
and if(![some method call]) ...
have the same cost - the "not" operator produces no extra code.