If it's for something like an airplane or powered glider, for best thrust efficiency and noise level, you want the prop on the front. If behind, the turbulence from the pylon causes both efficiency loss and a loud buzz as the blades pass through the pylon's wake. This is a bigger deal than the effect of the prop's wake hitting the pylon when mounted forward. The characteristic buzz sound that the Lake Buccaneer makes is from having the prop behind the pylon.
For an engine on skinny pylon with little frontal area, the difference between pusher and tractor is not that much, so noise effects are probably a bigger issue. The efficiency difference grows when the body in front of or behind the prop gets large. From an article by designer Peter Garrison in FLYING:
Now, how to compare this with the efficiency efficiency of a pusher propeller? For this I'm obliged to turn to books, since it's not possible to do a rule-of-thumb calculation from basic physical principles, as it was for the tractor. In Roskam and Lan, Airplane Aerodynamics and Performance, there is a chart showing the effect of what you might call the occlusion ratio -- the ratio of fuselage diameter to propeller diameter -- on the efficiencies of tractor and pusher propellers. The efficiency of the pusher prop is lower than that of the tractor, but negligibly so, until the fuselage diameter reaches half the prop diameter, at which point the two curves begin to diverge. When the fuselage diameter is 60 percent of the prop diameter the pusher has dropped behind by about 3 percent; by the time the fuselage diameter is 70 percent of the prop diameter, the decrement is 8 percent. An equation is provided (it is also found in Dan Raymer's Airplane Design: A Conceptual Approach) that yields roughly the same result.
The Volmer Amphibian is a good illustration of this. It had the option of doing either; you could design the mount to point the engine forward or backward. Part of the attraction of the tractor option was it didn't limit the engine choices (you need an engine with thrust bearings for pusher installation if you want to install it that way, as well as a prop designed for pushers).
Having the prop forward is widely seen as more efficient, with better thrust for the same horsepower and a simpler installation. I would say most Volmers were built as pushers however, mostly due to discomfort with having a propeller blade tip passing a foot and a half behind the pilot's head (I'd not be too happy having to put the thing into trees with that head chopper right behind my head, and if I was building one, there is no way I'd have the prop on the front).
The consensus however, seems to be that the tractor arrangement is superior performance wise, while the pusher is safer.
In the case of an electric motor glider (which I assume is what you are contemplating) there is another factor, which is the desirability of an aft folding propeller if the mount is fixed.
Recognizing that the efficiency difference may not be enough to worry about although the noise effects could be significant, if the pylon retracts, I'd put the prop on front. If the pylon is fixed and the prop feathers, I'd also put it on the front. If the pylon is fixed and the prop folds, then it goes on the back.
