I also have an iPhone and have tried a couple of the Korean dictionaries. My best suggestion would be the TouchDict by dreamwiz. It's definitely not perfect, but its a great dictionary if you want a Korean- English- Korean dictionary without having to buy multiple apps, and it also has Korean only definitions that will help make the transition to a Korean only dictionary much easier.
The Korean only dictionary I have is from DaolSoft and I think its really good, but I haven't tried any of the ones with English and Korean. There are just so many of them that you can't tell which is the best (and I for one don't want to spend all that money to find out) but the best thing that i can say is that the YBM has more entries than the Nexus, but they both seem to be setup the same (and they are more like translator dictionaries, they simply give you the Korean/English synonyms without giving background as to which word is more appropriate during certain circumstances). And both are made for Korean learners of English.