Jonathan, IMO it really comes down to what you will use the most in your compositions. Others also make good points about having a complete strings library as well as choosing the more expensive extended library.
In the end, as you said, having both would be beneficial. The library combination of SE, SE extended, AP Strings, and AP Strings extended is a fairly common one. Note that my interests lie in more traditional orchestral/chamber music and not film/epic so AP strings is not a first choice. However, with your interests, aiming for the above combination may prove to give the most long-term flexibility at the least overall expense.
In my own case, I have finally been able to start purchasing full libraries (through the great recent holiday promotions), and there is no question that I prefer them by a wide margin over SE. However, there is no chance that I would ever be able to afford all of the standard and extended libraries. My hope is to gradually acquire key ones, and use my various SE libraries for the rest. Three major advantages of the full packages are: greater number of velocity layers, chromatic sampling, and more articulations. If one can afford the expense, going straight to the full libraries is not a bad idea. For most of us, myself included, budget constraints simply eliminate that option. The SE libraries are a great value in that regard.