Hi again young man,
Once again, my knowledge of plethysmography is from an era when we used to perform the procedure "manually" and when we measured it using a physical electronic "pen" [rather like the pen of the old dot-matrix printers of the 90s lol) which you could see actually drawing the graph. In view of this (and please don't try to guess my age, because if you get it wrong, the whole of TSR community will run like hell haha), I am not conversant with the precise tech of the current digital plethysmographs; however, I would say there would be no significant effect due to distance from the heart being different, simply because you are not measuring any aspect of the force or volume of the remnant of the cardiac impulse at either the finger or the ear.
You are only measuring the change in local volume (of the WHOLE finger OR ear then assuming that any changes are due to change in blood volume, at least in our traditional instruments) - I suppose because the pinna of the ear has a relatively rich blood supply so the signal at the ear might be a little stronger than the one for the finger, BUT SEVERAL OTHER FACTORS WOULD HAVE A POTENTIALLY GREATER EFFECT ON THE SIZE OF THE SIGNAL e.g. the tone of the sympathetic/parasympathetic outflow to the finger or ear, which itself is influenced by several complex mechanisms including any anxiety [it is conceivable that a young subject would be a bit upset if you are sticking all sorts of gadgets in various corners of their poor bodies lol) which would change the levels of NA (noradrenaline) in the blood, as well as the effect this tone has on the diameter of the medium-sized arterioles (the so called resistance vessels) which In turn would determine the level of blood flow to the finger or ear, etc, etc etc,
Having said all this, it is opportune to say here that this point about signal strength is academic because the Q you originally posed was about the TEMPORAL RELATIONSHIP and NOT any AMPLITUDE BASED PARAMETER.
I hope I have stimulated your brain enough for you to join Cambridge Uni in 2 hours time lol! (although they might request you to hold your horses till October!)
M.