Before all that carbon dioxide could take effect during the Great Dying, the 75 trillion tonnes of sulfur aerosols released by the Siberian Traps actually helped to blot out the Sun for several decades at least. The planet entered a snap freeze, photosynthesis in parts of the world stalled, and food chains across the world started to collapse.
The most notable effect sulfur aerosols have ever had was probably during the formation of the most prominent Snowball Earth. Around 700-800 million years ago, there was a single supercontinent named Rodinia – Russian for “The Motherland.” When supercontinents have been assembled, carbon dioxide-effusing eruptions are generally rarer, which means there's not much to offset any massive sulfur dioxide discharges.
So, when a range of volcanoes 3,220 kilometers (2,000 miles) long erupted, the aerosols managed to block out so much sunlight for so long that for perhaps hundreds of millions of years after that, glaciers spread from the poles to the equator, and from a distance, the Earth looked like a giant snowball.
These are the extremes – and again, this sulfur is emitted over a very long timescale. If an eruption like that happened today, the warming effect of the GHGs would overpower the sulfur without a doubt.
In short, we are simply producing too much carbon dioxide, which renders both arguments null and void. Consider these myths debunked.