To address the 'ecology' side of your question, there are a few factors to consider:
- How much meat/fish you are eating in both scenarios - probably an obvious one
- How is the fish caught? Pole and line is the most sustainable method, as it minimises capturing endangered species, non-relevant species (dolphins, sharks) and damaging the ocean floor which methods like trawling will do. Farming may be better still from an environmental damage perspective, although I'd need to look into that further.
- How do you source meat, fish and plants? Switching to veganism reduces a lot of the greenhouse gases produced by livestock but if you're then eating fruit and veg that has a load of air miles, it's not going to be as big of a carbon saving as buying locally (although probably never enough to make it worse than eating meat)
- What plants do you switch to eating? For example, avocados are linked with large-scale rainforest destruction, and rice paddies produce methane. Similarly, sticking to oat milk uses less water than soy milk. Are you buying organic, or are these grown using pesticides? There are a few factors here although, again, probably not enough to be more damaging than eating meat
- What fish do you switch to eating? While cod is a commonly eaten fish, its wild populations are under significant threat from the fishing industry. Sticking to species like Salmon (often farmed instead of caught from the wild) and Pollock (which have healthier natural stocks) will be better from an ecological perspective. Better still, animals like oysters and mussels are farmed (so no negative impact on marine wildlife) and actually have a positive environmental impact due to their water filtering habits
- How is food packaged? Compare picking up stuff from a market to it being wrapped in cellophane from the supermarket. Plastic doesn't biodegrade and has significant ecological consequences. This also applies to restaurants and takeaways that you are buying from in both scenarios (I know anecdotally that vegan places are more conscientious when it comes to packaging)
- How is food stored and cooked? Think about the energy required to freeze/chill and cook food in both scenarios. This will depend on your location - I'm in the UK so fruit and veg don't need to go in the fridge or freezer the way that meat and fish would need to
I imagine a lot of these questions will be relevant for understanding the health impact as well, but I'm not as well educated on that side of things.
Broadly speaking, a pescatarian diet has a much lower ecological impact than being an omnivore, and a vegan diet has a lower ecological impact still. Given that 'ecology' has so many parts to it (environmental damage, carbon production, pollution...) there is no easy answer. However, while there is no 'definitive' answer, hopefully these things will help you decide a lifestyle that's right for you and sits well with your values.