I don't like manual steering just because of the ratio they use to keep the effort manageable. You just don't have the same control over your car, especially in emergency manuvers, with a manual box that you do with a much faster ratio power box. The effort once the car is rolling is not the issue but it sure is in a parking lot. Plus there is a huge difference in the effort requred to turn the wheel from the stock 6.95 tires and the 235/60 tires I am running. Also, the weight savings people talk about is not that great, about 25lbs.
If your car is going to be primarily a street driven vehicle do your self a favor and go power. Get a Firm Feel box or go smaller on the steering wheel diameter if your are concerned about the assist.
I have all the parts on the bench to swap my 68 Barracuda over to power, just haven't gotten around to doing it yet.