Which is better?
I am not an expert at all on this issue. Newer cars use electronic ignition operated by a computer. So this question only applies to older vehicles that have a distributor and no computer running anything.
What the issue is getting the proper timing from your distributor for firing the spark plugs at the proper time that your engine likes. Gasolene is not as combustible as you might think. In a piston when you introduce a spark to the air-fuel mixture, the explosion in the piston is NOT immediate. It actually takes a small amount of time for the spark to ignite the full mixture. The timing of when that spark first starts is important. You want the fuel to ignite after the piston reaches top dead center in the cylinder. If the spark is too soon or too late the engine will not run properly or optimally. Or worse you could have detonation which is a loud knock that could lead to engine failure. So there is an optimum point to do this. The timing set using the distributor affects this.
The engine needs different timing depending on the rpm of the engine. Distributors have mechanical timing advance built into it using springs that are affected by the centrifugal force. This advances the timing as the engine RPM increases causing the spark to happen sooner.
Where the question came from in the first place was just prior to pollution controls being imposed on cars. In the years before that, GM in particular, used ported vacuum on their cars to cut down on exhaust pollution. Ported vacuum offers no vacuum advance at idle, which increases the operating temperature of the engine which in turn reduces emissions.
Everything You Wanted to Know About Vacuum Advance and Ignition Timing
Evan Perkins MotorTrend Writer Jan 4, 2016
“There’s a tiny silver can on the side of most distributors that is easily the most misunderstood component of any distributor-based ignition system. Feared by many, and ignored by many more, the vacuum advance can is an important component of your ignition platform that offers both performance and economy. Leaving it unplugged is akin to throwing free engine efficiency straight down the drain.
To fully understand why the vacuum advance can is a necessity in any street-going car, we need to dive into spark timing as a whole and cover some ignition basics.”