For one thing, the amount of actual steroid in the ML of the different esters is not the same. You get more testosterone out of a single shot of 200mgs of Prop than you do out of a shot of 200mgs of Enth.
The other thing is that blood plasma levels of an equal amount of Prop vs/ Enth are DRASTICALLY different. Check out the "Minto Studies" which are on Deca, but give you a lot of info about (similar) Esters, i.e. Phenylprop vs/ Decanoate (my Deca profile on steroid.com has all the graphs)...
When shorter esters are used (phenylpropionate vs. decanoate) there is a higher peak plasma concentration of the compound. In the case of the minto studies, nandrolone phenylpropionate caused an almost double peak plasma concentration when compared to nandrolone decanoate.
You'll note from those studies that injections in larger muscle groups (glutes vs. delts) yields higher blood plasma concentrations of the compound injected, when mg levels are the same. So you should always shoot preferentially in large muscle groups.
Also, greater mg per ml yields greater blood plasma concentrations when total mg amount is the same. So try to buy more highly concentrated gear when you can.