Anna or Belle ?
Never used the E5 or E6, so I can't comment on that ...
however, the issue about Anna vs Belle - I am currently using the Nokia N8 on Nokia Belle, Belle increased overall performance management for Symbian, thus the issue of Anna vs Belle is moot. If you run Symbian, my recommendation would be Belle, hands down. On my N8, my memory NEVER drops below 48% (~ 123 mb FREE RAM) when idle & I have had NO memory issues since updating to Belle. When I used Anna, memory usually hovered around 100 mb FREE, sometimes dropping ~ 75 mb free!
I will say, I use Swiss Manager Pro (which kills latent background apps, user defined !) which helps w the overall performance - but, since Belle - I have had NO performance issues.
informed/intelligent comments are welcome.