I'm surprised you didn't see much difference in terms of speed between the two. May be 5 days wasn't long enough or may be your GS3 was faster than mine for some reason (mine is on 4.3)
Some Touchwiz things are really nice, better than stock android imo. I like the functionality of Touchwiz dialer - speed dials are there (am I the only one using it?

), the call log shows date and time, not how long ago calls took pace. I'm sure there are plenty of dialers that have have the Touchwiz features and stock android looks, just need to look for one.
In my case N5 would have been (almost) perfect if not for the microphone issue. You have to hold your phone close to your mouth in order to be heard, or use speakerphone. GS3 microphone works exactly the same regardless whether speakerphone is on or not, while the noise cancellation on GS3 is excellent. When I'm in meetings that last hours the phone is always on my desk, about 1ft away from my mouth, I never ever use speakerphone and everyone can hear me well. With N5 speakerphone or headset must be used or I'd have to hold it 2-3 inches away from my mouth.. else nobody will hear a word.