[Gav]

It is completely dependant on what it is that you want to do.
If you are after a cruiser and not a track car 3.5's would be the way to go.
If getting off the line in a hurry is your thing 3.9's are great.
I have 3.9's and would not consider the 2000rpm mine does @ 100k's too high and I run an auto with a higher overdrive ratio than the T56