The discussion whether the one is killing the other is getting hotter and the opinions are raging high.
First of all these two “opponents” are hard to compare as friendfeed being a information aggregation tool and twitter a hybrid between instant messenger and micro blogging device.

Both serve certainly completely different needs, twitter mainly focuses on spreading rather personal opinons/feelings/emotional states/gossip, only randomly establishing a real conversation.
Friendfeed resembles a honey pot of information gathered by its user acting as honey bees collected from various flowers, like Digg, GoogleReader, del.icio.us, stumbleupon and many more.

Definitely the value of information might be varying, but like on twitter it users can adjust that by choosing their subscriptions.

Looking at the user stats both are steadily growing with twitter being comfortable in in the lead.
One of the reasons for this distance is certainly twitter started in August 2006 and friendfeed in October 2007. Another cause might be twitter is used for communication, friendfeed on the contrary deals in information. This shows why the geek ratio on friendfeed is definitely higher…

As a result both services will mostly not bother each other in the future, with their target audience being to different and having unequal demands.
So Twitter vs Plurk or Friendfeed vs GoogleReader would give much more challenging and interesting fights…