Low-E is actually more sophisticated. Different rating are designed for different areas of the country. In CA, they even sell different rating base on where you live and your requirements. We have lived with low-E now in SF Bay region for 3+ years and we can share our experience.

If cost is a real concern, then you may have no choice. If you can afford the cost but just want to be sure you are buying something worthwhile, then I suggest the following.

Ignore the cost, and pretend you are buying low-E. Then consider where and how low-E can be applied to your home. How it may change usage of certain areas in your house. How lighting may change, how curtain or window treatments may change. Do a compare and contrast. When all is done, you can bring in the cost factor and see if the gain is worth the cost of low-E.

We have a bank of French doors facing SW and a bank of large Casement facing S. in the winter, the sun is low enough to shine right at these doors and windows. It heats the house to about 70 by noon, from 60 at sunrise. It warms to 75 by 4PM and begins to cool at sunset. It drops down to 69-F by 9PM. When a winter high pressure settles over CA, it can get hot enough inside that we open windows to cool off the house. We never run our heater on sunny winter days. On cloudy days, we have to run our heater but it runs short and heats up fast. The house maintains its warmth easily due to low-E. All this, without any curtain or window treatment.

We looked at low-E in 1991 and it cost $450 extra per window so we did nothing. We lived with our old Aluminum dbl-pane clear window. We spent $6000 on exterior solar shade and another $3000 on interior Hunter Douglas triple cell blinds. We thought that would address the heat and cold problem. The reality was the house was dark and gloomy on hot summer days and yet it baked to 80 and 85 and the A/C cannot keep up. The house was also dark and gloomy in the winter and yet it routinely dropped to 50-F and we had to run the heater all day. And lights are on year in and year out because the blinds and shade are always drawn. We did that for 10 years and it was horrible and foolish in hind sight.

Now, natural light comes in all year, the house is bright and airy and we rarely run our A/C and only run our heater on cloudy days.

Good luck in your analysis.