I do only own the E906 and have read up on the 606 a bit while buying.
From what I figured the 906 is the better option cause it generally sounds a bit better to most ears and has the added three way switch.
Now you might be scared of that extra option, but donīt be. It really is very usefull I find.
I am speaking mostly from a live perspective here, as I used the mic mostly there, but I found a point where to place the mic which is so that the cage is just about all on the membrane and then I just need to use the switch to go between normal and boost, depending on the amp, to get a very usable sound in the location I usually work.
If you donīt wanne use that for recording, fair enough it is after all just another EQ, so you might leave it on the mid position 90% of the time, but then you still have a bit better sounding mic (there are comparison clips somewhere out there) and sometimes you can use a combination of the switch and positioning, which really is very easy, to get the best out of a odd sounding amp without too much trouble.
My 2 cents at least.