I was faced with a similar decision as well, but I reasoned it out to as follows:
Price: KJW and TM are equal in price. However, you get a FMU kit with the KJW, and even though it doesn't have trademarks, it is still a major plus. If you were to get the FMU kit separately, then expect to pay at least another 100-150. The G&P kit isn't top notch, but it serves the purpose well.
Performance: KJW rips the design from TM, literally. Most, if not all, the internal pieces are interchangable. Performance wise it will be similar. However, the TM mags do not fit the KJW. If you dig up Madmax's thread on KJW fill valves, you will see that they don't fill up very well either. Again, this relates back to the price. Pay more and get more shots per mag, or just carry around a can and fill it up out on the field, or even buy another mag to boot.
Reliability: Luck of the draw on this one. With TM, you're guaranteed a working gun that will last you for years as long as you don't tamper with it. With KJW, you may need to fiddle around with certain things before you get it to work perfectly.
As for your duster vs propane concern, it is a moot point. The TM with a FMU kit will not function very well if you were to use duster as the propellent. You would need to change the valve, and strengthen the hammer spring. Even then, duster may not provide enough power to cycle the slide properly during rapid fire.
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