Which is the simplest way to transform +9V and +5V into 3.3V CMOS acceptable digital levels?
simulate this circuit &ndash; Schematic created using CircuitLab

Try simulating the above schematic.  The R1/R2 voltage divider will reduce the signal that is either 5V or 9V to a lower voltage.

9V –> 3.5V @ VA

5V –> 1.9V @ VA

So when it is 9V the 9V will bias the R1/R2 so that VA is higher than Q1 emitter and Q1 will be off.  When it is 5V it will allow VA to lower, turning on Q1.  There is about 76uA coming from R3 into VA but it's impact is small compared to about 1mA of bias through R1/R2. While not super-precise, it does do the job with only a few simple components.  If whatever PNP transistor you have doesn't have enough gain, ie. doesn't fully switch on, you can increase R4 to 3k3.