<p>Lets say I've got two interchangeable pieces of code and I want to figure out which one of them takes less processor time to execute. How would I do this?</p>

<p>To get a very rough estimation I could just put NSLog() calls on either side of the code I wanted to profile, but it seems like the processor being otherwise very busy could skew the results.</p>

Unless one of these two pieces of code is already in your app,
and
you've already profiled your app's overall performance to determine that the existing code is a major bottleneck, then what you're doing is called "premature optimization."
Xcode includes an excellent
profiler
tool called "
Shark
." For some versions, it's found in /Developer/Applications; for others, it's in a "Performance Tools" subdirectory.
Shark will tell you exactly how much time, as a percentage of overall execution time, your app is spending in each part of your code. The idea of using a tool like Shark is to follow the "80/20 rule" - your app will spend 80% of its time running 20% of its code, so for the best results you should spend 80% of
your
time optimizing that same 20%.
So, to answer your question directly, assuming that you
have
run Shark, and you're looking to optimize the topmost bottleneck, simply replace it with your optimized code and run your app under Shark again. Then, compare the percentage of the overall time being spent in the replacement code to that of the original.