There are some things I feel Cortana does that Siri and Google Now does.
Every morning Cortana gives me the weather, appointments on my calendar, news stories and how long my commute will take given the current traffic conditions. It also learns when I normally leave for work, so about 10 minutes before I usually leave, I get an update on how long it will take to get home given the current conditions.
To me, Cortana is more like an assistant. You can set stuff so its off limits and she will ignore, and likewise, tell her to remember certain reminders like you pointed out.
Do either Google Now or Siri have these features that actively learn stuff about your preferences?
Things like these are a much more useful part of it than voice detection, I very rarely interact with it via voice, and tend to just do searches through my normal browser. It also hooks into Gmail and can give you reminders about things like flights you've booked, although that raises questions about privacy, and whether you really want Google to be trawling through your emails.
Eh, it also does it at really inappropriate times.
About 30 minutes after I get into the office it starts telling me how long it'd take me to get home, and that card stays up in Google Now until I actually do go home!
I haven't used Cortana, and have only had disappointing experiences with Siri (after coming from Google Voice Search); how does the performance on Google Voice Search Compare? I wonder if the performance rankings are just a complete reversal of the quality rankings; If so, I wonder who's closest to the sweet spot?
Have you tried it on iOS 8 since it came out? They finally added streaming voice recognition, which speeds it up a lot in my experience. Previously, it waited until you were done speaking to initiate the upload, which added a pretty big delay to any response.