From a simple physics perspective, please consider the following. A fan or blower located in or near the window will pull cooler air into the room. Ideally, you have to have a means to exhaust the ambient air, allowing such airflow. This would require an exhaust portion in the window partially separated from the fan. As you've noted, hot air is in the upper portion of the room, making the top of the window a better location for the exhaust feature, either passive or another fan. This means the answer to your question about which direction is better is that both are required.
The difficulty arises in splitting the airflow. It is practical to consider that the incoming air will shed some momentum and reverse course and exit the upper portion of the window.
Your best solution may lie in the ability to create baffling to direct the airflow for your comfort. A baffle could be something as simple as a lightweight wooden frame covered with paper, easily moved, not particularly expensive. Use the baffle(s) to split the incoming air, specifically around the area of the sleeping plank.
If your budget allows, small (personal) fans attached to the baffle frame can be used to further guide the airflow to provide assurance of comfort from the cooling air.
For a living space, it seems impractical to create and install ducting from the window to the sleeping area, although such a construction would provide for the greater temperature management.
If your living space allows, suspending paper frame diverters from the ceiling on light rope or strong cord would also allow for vertical management of the airflow. Such an installation could be "dressed up" as an art piece improving the overall appearance.
Even if you lack the materials or tools to construct a light wood frame for paper, you should be able suspend from lines some paper panels to help direct airflow. Folding the edges over and applying glue will create some stiffness. Adding a layer of inexpensive cardboard would improve this, especially if the cardboard was attached edge-on to the paper with the corrugations appropriately aligned to enhance stiffness.
Obviously, if you are unable to suspend from the ceiling any of the constructions, another method must be considered.
The foundation of all this mindwork is that you have to have a means of moving the air in and moving the air out and keeping the two paths separated and aimed in the desired directions.
EDIT: additionally, you can have a bit of fun at the same time as determining the usually invisible airflow. Purchase a helium balloon of mylar construction. Ordinary plastic balloons have a too-short lifespan for this aspect of the project.
Attach to the balloon string as closely as possible a paper cup. Either cut the excess string or place it in the cup. Add easily removable weight to the cup until the balloon neither rises nor falls. In the past, I have used balled up pieces of newspaper. Also with a foam coffee cup, one can break off small sections and return some to the cup as required.
With the balloon in a neutral buoyancy state, it will follow airflow in the room quite well. The above configuration should be performed when the windows and doors are closed. When your selected airflow director system is in place, the balloon should be manually placed in the incoming air. It will show you clearly whether the airflow will reach the sleeping area.
If you discover, for example, that the airflow diverts under the plank or to one side or another, you will know with more certainty where to place a diverter.
