Handling DB 5 vs DB 6

Quite a lot better.

It's not just the longer wheelbase that helps, but that the rear suspension arms are a lot shorter. They longer ones of the DB5 tend to flex a lot.

The real clincher though is that the DB6 brakes are a lot better. You do take your life in your hands with a DB5 sometimes.

And at higher speeds, the DB6 doesn't lift as much as the DB5 due to the better aerodynamics. Above 100mph the DB5 lifts quite alarmingly and the steering goes very light indeed.

How do the 2 Astons compare to an early 911 ?

An early 911 will handle better (until it throws you into the hedge), but the Astons have anything up to 200bhp more power for only a slight weight penalty so they're much faster in a straight line.

I was at Le Mans with the Porsche club a few years ago in my Aston and when we were driving about in convoy it shocked everyone just how easily the DB6 pulled away from the Porsches on the straights. You needed a very modern 911 to keep up.