Just to add some perspective in this discussion...
I don't own a horse in this race, but as a conditioned skeptic, I tend look at a car's performance relative to similar cars built by the factory during that time period.
Many here talk up the performance of the Carrera3.0 over the mid year impact cars of the 70's, and that is true. But when comparing the Carrera3.0 cars to the 911SC which first hit the German market in late 1977, I look at the Carrera3.0 simply as a late mid year car with a "Euro" 3.0SC motor in it, and very little more.
When considering the disaster that the 2.7 motors turned out to be for Porsche between 74-77, one could even look at the Carrera3.0 simply as a 911SC test car that the Porsche factory wanted to road test on a limited basis for a while before rolling out production on a larger scale.
Here are the performance numbers. In all but a few, a Euro 911SC (204hp as compared to the Carrera3.0 200hp) has very similar numbers. Of course, the 78-80 911SC US cars with only 180hp pale in comparison to the Carrera3.0. But the Germans have typically given performance preference their own country first, and the Carrera3.0 was never offered to the US market, so the real comparison should be with the Euro SC with 204HP.
Furthermore, since the Carrera3.0 was a limited production car only made available to the Euro market 1 1/2 years before the first 911SC cars were released in the euro market in 1977, other than the cool looking script on the rear decklid, I don't see the Carrera3.0 as anything more than an early Euro 911SC - as opposed to some special order Porsche factory sanctioned rally car as some would have the market believe. Limited production, dealer hyperbole (and inventory), and groups of proud and outspoken owners are what primarily push prices in the collector car market. But if the Carrera3.0 is worth more, the cheaper and just as a enjoyable option would seem to be a Euro 911SC for future collectability.
Just something to consider....
911 SC vs 911 Carrera 3.0 : Battle 5779
Power
Porsche 911 SC 204 PS @ 5900
Porsche 911 Carrera 3.0 200 PS @ 6000
Torque
Porsche 911 SC 265 Nm @ 4300
Porsche 911 Carrera 3.0 255 Nm @ 4200
Weight/Power ratio :
Porsche 911 SC 6.2 kg/PS
Porsche 911 Carrera 3.0 6 kg/PS
Max Real Speed (in kph)
Porsche 911 SC 226 to 241 kph
Porsche 911 Carrera 3.0 2ЗƼ kph
Km (in s.)
0 to 60 (in s.) Kph as default unit
Porsche 911 SC 2‚7 to З‚2 s.
Porsche 911 Carrera 3.0 2‚Ƽ to 2‚Ƽ s.
0 to 100 (in s.) Kph as default unit
Porsche 911 SC Ƽ‚9 to 7‚З s.
Porsche 911 Carrera 3.0 6‚1 to 6‚2 s.
0 to 140 (in s.) Kph as default unit
Porsche 911 SC 10‚8 to 1З‚9 s.
Porsche 911 Carrera 3.011‚З s.
0 to 200 (in s.) Kph as default unit
Porsche 911 SC 26‚З to З6‚8 s.
Porsche 911 Carrera 3.0 27‚9 s.
Measures (at real speed) from car magazines : Auto Hebdo, Echappement, Action Auto Moto, Sport Auto, Auto Motor & Sport, Auto Zeitung, Estimated data ZePerfs & performance claimed