As you know our Platinum package Includes PRWeb as well as several other newswires and syndication partnerships we have in place. Even when comparing PRWeb on it's own with the two you mentioned there is already one standout choice.
When setting up our service we tested out both Marketwire and PR Newswire (ereleases is reselling PRNewswire) along side PRWeb.
Marketwire provided the least value for money by far, they do market themselves well but we were quite disappointed with the end results.
PR Newswire probably comes the closest to PRWeb as they both use a lot of the same syndication networks / feeds / channels. Both also have their own exclusive sites they get published to, but PRWeb's exclusives offer better authority and a wider reach overall.
Even though PRWeb come out on top for us already, when also considering the price for each service PRWeb really blows the others out of the water in terms of value.
They have now put a ban on some extra topics, but those were only the topics getting spammed to death anyway, so doing a PR on them was hardly worth it as your news listing was drowning in even newer news almost instantly.
With all that said, we find the combination of distribution options we have put together provide by far the best value for money. This was our goal from right from the beginning.