The easiest way to choose between these is based on theme. For as many of these iPhone RPGs as are available there are relatively few different story components. The most common is the mafia theme, with games like iMob Online, iMafia, iMafia New York, Mafia Wars, Mafia LIVE (which usually requires you to pay for a full version) and others.
The next one is a racing element, which is not to fool you into actually believing there is any racing section to them whatsoever. iRacing and Race Wars are two very popular iPhone RPGs in this series and use the same set up as the others while switching everything around to include a professional and street racing theme.
Vampires, and other night creatures take another preference with iVampires Online and Undead LIVE, but are still the exact same format as the other iPhone games. There are a few, notably Girl Wars Online and Famous, that really escape this themed format and strike out with a little irony and social commentary to create a more unique experience.