Hi,
Both of these degrees are fascinating subjects and will help you develop skills useful in the workplace. In terms of careers, environmental science is a broader subject and may keep your options more open. I study environmental science and we've covered modules ranging from GIS to hydrology, atmospheric science, aquatic biogeochemistry, environmental radioactivity and hazard risk management.In contrast ecology and conservation will supply you with a more specific skill set and may give you a clearer route into a specific job such as a conservation consultant or a field ecologist.
Personally, I'd consider which of the two degrees you are more interested in and will enjoy studying the most for three years. This is so important for motivation and consequently achieving good grades. Have a look at the programme catalogues for different courses and see which modules interest you more. Also, if you have a particular career in mind see if it requires a specific degree title. In my experience many jobs in the environmental sciences only require a 2:1 or above in an environmental/earth sciences degree or even just in a STEM subject.
Hope this helps,
Becky (Lancaster university student ambassador)