Have a look at the notes for each of the coffees on BeanBay as they are all quite different. But in general the natural processed will be more fruity and the washed more choc/caramel (really big generalization though!)
Harrar is more berry (typically) than the Biftu which is more stone fruit like peaches. Yirgacheff is very citrusy and lemon - which gets a great wrap here on CoffeeSnobs but personally I am not a fan in espresso.
If you want something choc/caramel give the Columbian Volcan a go - that with the Biftu are my two favourite beans and always winners with guests.