Stability is a subjective thing and also highly dependent on condition. Of course with any car you want all the bushings to be up to snuff, and the front suspension links on 356s are prone to wear, but relatively straight forward to rebuild. The rear weight bias in the 911 and suspension bushings in the early cars can make them a handful at high speed, coupled with the low turns ratio rack and pinion. The 356 is kind of the opposite, with a smoother and more forgiving steering box and suspension with less weight in the rear, but more tendency for oversteer at the limit. So going down the highway they feel more settled and comfortable, but never lift throttle in the turns. The 911 is more stable in the turns but feels more high strung and demanding going straight down the road.
SV