Who is the more annoying?
Navi is infamous for her well known catch phrase "Hey! Listen!" Fans of ocarina have suffered many sleepless nights because they cannot get navi's continuous shouts at link out of their heads. Most of the time, navi is used to lock onto enemies. Other than that, she is usless.
OR IS SHE???
when playing ocarina the other day, navi's usual "hey!" came up. However, instead of telling me usless info that i already know, She told me where i needed to go. if i hadn't already known where i was to g

midna is by far the more interesting character. Her personality throughout the game goes from being rather controlling to being more kind. Midna does not help you nlock on, but does help you to cross large gaps in wolf form, and to quicktravel around hyrule. This makes traveling much easier.
HOWEVER, Midna tells you where you need to g

tell me something i dont know.
So to sum up
Navi-far more talkative but with useful information
Midna- Cooler than navi... but when it comes to guidance she falls short. (not a joke about her being an imp)