For a definitive answer, you would need to forward me the specific products that you priced and the exact window glass you have, but I can explain to you the basic logic on the efficiency of the two.
The one factor that will determine your overheating problems and AC bill is the percentage of the energy of the sun rays that makes it into your house. (If you would be buying new windows, this would be found on the label as the SHGC value.)
So let's look at what happens to a sun ray's energy when it hits your glass: a part gets reflected (good; that heat is out of the house), a part gets transmitted through and heats up your house, and a part gets absorbed, heating up the glass. The glass then transmits this heat both to the inside and the outside.
Why am I telling you all this? Because it explains that the further outside you block the sun ray, the better. External screens do a much better job than internal screens because the reflected sun ray (that already passed once through the glass and heated it up) does not have to go through it again.
An internal screen that reflects 60% of the energy will not make your house heat up 60% less. My guess would be it reduces the incoming heat by 20-30%. The energy an internal screen absorbs, if it's not white or silver, will also be radiated into your room. An outside screen, however, that reflects 60% will reduce your heat gain by a full 60%, and any energy it absorbs will be left outside as well.
Solar film also works better if applied on the outer layer of dual-pane glass, for the same reasons. The manufacturer can tell you how much energy it reflects, and how much it absorbs. As I mentioned above, much of the absorbed energy will end up in your house too, so do not just look at the figures they give you for transmitted energy. Solargard, for example, claims that only 12% of the energy will be transmitted, but looking at it in detail, 38% of the energy is absorbed.
In the case of single-pane glass, about half of that will go inside, so rather 38% divided by 2 equals 19%, plus 12%, for a total of 31% of the energy goes into your home.
Three other things to consider with films: you cannot easily remove them in winter, so if you like the sun's warmth in winter you reduce this too (being in Texas, this may not be a big concern to you). Also, the energy the film absorbs into the glass heats up the glass and may damage the seals on dual-pane windows, and you may lose your window warranty. Finally, all the manufacturers' calculations are based on single-pane glass, because that's where the film makes the biggest difference -- the better quality your glass the less film will help.
As you can see, the definitive answer is not available, at least not without knowing which products you are considering. But an external screen will beat any other option.
For more information:
You should also read Andrea Foss's Ask A Pro Q&A, "We're considering having a window film applied as a low-cost way to save energy. How effective is this?"