Tren is tren and test prop is test prop. People use the the cattle/hog pellets syno and fina to make tren and test prop because the cattle pellets are legal to buy, therefore easier for some people to get their hands on than ready to inject/eat/apply topically steroids. Doing the conversions is a pain in the ass though, and with the pellets used to make test prop there is the complication where estrogen also appears in the steroid conversion. Men want to avoid taking in estrogen so they don't grow tits. So doing this conversion is very time consuming in making sure you get as much as possible of the estrogen OUT. The way to tell is by getting the melting point of the powder because test and estrogen have different melting points, you want to get your powder as close as you can to the test prop melting point. A simpler way to do the conversion is to test base, that way there will be no estrogen in the powder. The downside to doing that is that test base/test suspension needs to be injected twice per day and the injections are very painful.
So I believe that most people would want to get their hands on steroids such as test prop in powder form so you don't have to go through the tedious and time consuming process of breaking down the pellets and then doing several washes while taking the melting point. When you buy test prop powder it is pure test prop. Or when you buy test prop suspended in oil it is the finished product ready to be injected with no work involved in preparing it at all. It's more expensive but also much more convenient.