We sell both and the PWK has a higher profit margin but I believe you will be much happier with a properly tuned OEM TM38 or a modified TM38 than the best tuned PWK 38 to PWK 41.  Also the PWK is approximately 10mm shorter than the TM 38 making it more apt to pull out of the the old hard rubber air cleaner or reed boots. 

I have tuned both carbs on the dyno and in the field and prefer the TM38 for the 500 Quadzilla.