Transistor Placement?
The two configurations are:



simulate this circuit &ndash; Schematic created using CircuitLab
The left side configuration is the common emitter

The input needs to be just 0.7v higher than the emitter in order to turn the transistor on.
A base resistor is required to limit the base-emitter current to safe levels (but also enough for the intended output current).
When the transistor turns on the load (led+resistor) gets the full power supply across it reduced just by the Vce saturation voltage.
A change in the power supply level also changes the voltage across the load.
The collector sinks current (provides ground) when the input (base) is positive, so the output level is inverted (the led on/off operation is not inverted).

The right side configuration is the common collector or emitter follower

The input needs to be 0.7v higher that the intended emitter voltage, so the proper drive level depends on the intended output level.
When the transistor turns on the voltage applied to the load (led+resistor) is 0.7v lower that the voltage of the base no matter if the collector supply level changes (assuming it doesn't become too low).
There is no need for a base resistor (it's optional), the base-emitter current is limited as a function of the required load current and the transistor.
The emitter sources current (provides positive supply) when the input (base) is positive, so the output is not inverted.
The power dissipated on the transistor can be high since it's the output current multiplied with the Vce voltage which changes as the collector supply level changes.


For a simple led you can use any of the two configuration assuming that a control voltage of led Vf (forward voltage) +0.7v is available.
In the common emitter circuit, because of the way that the base resistor is calculated (using a gain estimation) there will be some base-emitter current wasted (an emitter resistor is an option of course).
On the other hand with the common collector circuit the base current will be exactly as high as it needs to be based on the load requirement and the transistor gain.
If you want to keep the led voltage constant when the power supply level changes then the common collector is better suited.
If instead of a simple led you have a seven segment display then the common emitter suits better to drive a common cathode type and the common collector to drive a common anode display.