Portland Cement is usually in a better moisture resistant package than the pre-blended mix. Despite the volume used, the price of the bag might almost be equal to the cost of the materials in a good bag. Pre-mixed is made for the U.S. price sensitive market and retailers that may have a quck inventory turn-over.
Portland could have what appear to be lumps in the corners due to the storage conditions and weight above and they are not as bad as they look and it may be usable. The contents are 100% dry and the bags ar filled more more valuable materials, so there are more precautions. Pre-mixed may contain some moisture in the aggregate that can cause hydration and "hardening" of the contents, it is wise not to store too long and always toss an opened bag.
The bottom line is that if you you buy ready-mix from a truck, it is controlled, uniform and can be cheaper in the end even with a short term surcharge if you consider cost, time, wear and tear to buy too much and then have to get rid of the excess.
Dick