There seems to be a lot a confusion about M1 EP, which is understandable. I am just a consumer, but was interested in this subject since Advance Auto Parts had them on sale for the same price, so I did some research and here is what I found out (or what I suspect):
1. The M1 EP formula changed a few years ago. Originally it was suggested by Mobil that EP could go up to 25,000 miles, but it was not API certified for most new car warranties, and Mobil specifically recommended against using it for engines still under warranty. After they changed the formula it now does meet GF-4 and API SL/CF specifications required by most new car warranties (except for M1 EP 15-50 which is a different animal), and of course it is guaranteed up to 15,000 miles (interesting to note that some other boutique oils that are guaranteed to 25,000 miles are not GF-4 and API SL/CF certified).
2. Mobil 1 EP contains 50 percent more SuperSyn than regular Mobil 1. What is SuperSyn? That is obviously proprietary, but I think it means that EP has a little more PAO/Ester and other boosted additives that are part of the SuperSyn package. The term "boosted" comes from this spec sheet:
http://www.mobiloil.com/USA-English/MotorOil/Files/Mobil_1_Product_Guide.pdf
3. There are the usual M1 bashers on this thread who say the difference is only marketing, but that is obviously bunk. The difference is not huge, and the price difference of a 5-quart jug at Walmart is not huge either, only $4 (used to be only $2.50 difference before the recent Walmart price drops). But if you are going for a extended OCI (or are willing to skip a few Biggie Fries and Biggie Drinks), it probably is worth the difference in price.