The catch with the Epic Collections is the volumes aren't printed in chronological order, for example there's currently four Amazing Spider-Man volumes available but they are volumes 1, 15, 20 and 22 (with Vol 21 out early next year). The majority of the other titles in this series follow the same rule of out-of-order releases.
The Masterworks are really nice, but can be released infrequently, contain less issues and go for a higher price. As they are always released chronologically, you will generally be able to estimate when a certain run/story will be reprinted. You can't really do that with the Epics.