Friedel was our first well-rounded top-flight keeper. Remember he was voted best EPL keeper once (as was Howard in his first year at Man U). Keller is highly one-dimensional in the sense that he is the best shot-stopper we have ever produced (especially at very close range), but horrible with his distribution. So, Keller is great against certain teams, but not as much against others. Anyway, in this cup, we don't have the personel to switch keepers for different opponents as we could have in 2002, so it's Keller or bust.
To answer the question directly though, in 2002 Friedel was better (and many would say is still better today, but that's debatable).