I think pescetarianism, when done properly, is most likely better than veganism or vegetarianism in terms of reducing animal suffering and environmental impact.
-With veganism, almost 100% of human calories from the 7 billion people on this planet have to come from land, with the exception of the negligible amount of calories humans consume from seaweed. The oceans are 2/3rds of the world's surface, which is largely untapped when eating vegan. Pescetarianism, however, distributes the calorie consumption over the world's surface, meaning there is less concentration on land. Over-fishing is a practical concern, but when done properly, can be largely avoided. The Alaskan wild fisheries, for example, are considered some of the most sustainable in the world due to tight regulation. Fishing quotas keep the fish numbers intact, rather than allowing the short-sighted practice of over-fishing followed by periods of low fish counts. Augmenting land-grown food with sustainable fishing practices relieves a portion of the environmental pressure from the land.
-Eating plants, in a modern context, kills animals. One study estimated that for each hectare of wheat farmed, an average of up to 100 mice die. Then there are snakes and other animals unaccounted for that die also. And then there are countless insects killed from pesticides as well as the harvesting machines. And then a lot of the fertilizer drains into the soil, into the rivers, and eventually into the oceans, where it creates deadzones where fish die over a span of thousands of square miles. To reasonably compare life loss, we'd have to, for example, take an example of one wild-caught salmon that can provide 20 fillets (worth thousands of calories total), to the equivalent calories of wheat or another crop, in terms of how many mice, snakes, bugs, and fish die from the harvesting equipment, pesticides, and aquatic deadzones. Or, for example, there are mussels. Mussels can be farmed along coasts, and when done properly, are considered healthy and sustainable by sources such as the Monterey Bay Seafood Watch. Mussels are very simple creatures, probably comparable to insects, so one could compare the deliberate farming and consumption of mussels (which are healthy and a good source of protein), to all the countless insects that die as an uneaten byproduct of crop farming.
-Transportation has to be taken into account for all forms of diets- vegetarianism, veganism, pescetarianism, and omnivorism. All else being equal, a local diet is preferable, but sometimes not all else is equal.
And then as Mystic pointed out, omnivorism when done correctly can have certain advantages over vegetarianism and veganism. Nature is itself a balanced system. Animals fertilize plants, and the plants feed the animals. When humans separate the two, and farm only plants, and then farm animals in a big factory farm, they eliminate the balance and create problems. Suddenly, the waste from the animals becomes a problem, and food for the plants becomes a problem, whereas in nature, the problem of one becomes the solution to the other. If only plants are farmed, then a problem is how to get food for the plants. Fertilizer is needed, generally has to be shipped in, and runs into rivers and eventually creates deadzones in oceans that kill sea life. Plus, the world has an ongoing problem of shrinking arable land from intensive farming practices; trying to get as many crops as possible from the land in order to feed the masses and pay the farmers, and the cost of reducing soil quality and draining the nutrients out of it faster than they replenish. Some small farms simulate nature as closely as possible, where they have animals and crops that work in a loop that balances itself out fairly well.
That is why I switched from many years of vegetarianism to a more omnivorous, and typically pescetarian diet. I believe it to be healthier, and when most variables are accounted for, better for the environment and for animal suffering, compared to vegetarianism and veganism as normally practiced in a modern context.