Having spent excessive time in my friends 06 before buying my 07.. I can say the tower shake is non existent on the 06 it's a double ladder design at top. My 07 before I put the tower braces on was just crap. After the braces it's perfectly fine. I would not worry about this in your decision.
the 6.0 or 8.1 is the only way to go though the 06 had the MCX and it was a dog compared to my 6.0 which is also MUCH better after a prop change.