As whiskas says the main diff to the user is no email and no webspace, plus lack of content (not that will bother most).
The diff you dont see is the support, Openworld has far better than No Frills.
Its £3 less and just for pop3 this makes it worth it to most.
TBH, No frills are lying twats when they sell and seem to get away with mis-information all to often, I can speak first hand here and say the amount of work Openworld has to do, to make up for No frills lying is amazing.
There is more i could tell you but im afraid id get into trouble... trust me though, stay away from no frills unless you like crying in the end.