Depends on the reason behind being vegan / vegetarian. If it's to reduce animal suffering, eating chicken is almost certainly causing a greater amount of animal suffering than if you eat cows.
I find the approach of the cited web page very strange. Basically, it says, the longer the animal lives, the more it suffers. Throws in some arbitrary multipliers, divides by mass, and voilà.
I would not call healthy animal life as suffering. Details matter. We all have seen peaceful cows and healthy fish. What counts, IMO, is fear, stress, pain, and the way animals are dying. That's not accounted in the article.
One may argue that the number of lives taken counts too. And then eating beef or whale meat will kill fewer animals than eating chicken or insects (of we don't count what whales eat). But in my opinion eating insects would cause less suffering.
Overall I think that eating smaller animals more frequently is more natural and oppotunistic. And eating cheaper meat will reduce overall carbon and water footprint.
From the welfare perspective, if the cows are just out in a large pasture, they will likely have net-positive experiences across their lives. But there are animals that have overwhelmingly negative experience throughout their lives (pigs confined to a cage where they can't turn around). As for chickens, >99% of which grow up in cramped conditions, with no access to the outside, with selective breeding causing them to grow so fast they often can't move, it is better to never have been (their lives are a net negative experience).
It's unfortunate, with respect to emissions, cows are worse than chickens, but with respect to suffering on sentient creatures, chickens are worse than cows.
Great news! There's a solution: eating fewer of both.
https://reducing-suffering.org/how-much-direct-suffering-is-...