It depends on a couple things. What you want to do with it? And how much are you willing to spend?
In a WD45 if you're looking for rarity and power, go with a diesel. The diesels have a 6 cylinder engine compared to the 4 cylinder gas. They will be more expensive though. Other then that, like what was said earlier, just make sure your getting a 45 motor in a gas tractor. I think LPs are rare too but don't know much about them for comparison against gas.