Both the disciplines serve different purposes. For example, the production of human insulin using Recombinant DNA technology is a good example of the importance of biotechnology.

Environmental science is the field, studying which you can understand the possible dangers of using GM crops for agriculture (these crops are prepared using biotech tools)

Advances in biotechnology can be applied to address environmental issues as well. E.g. GM Pseudomonas used for tackling oil spills.

Biotechnology offers you jobs in corporate companies involved in developing and marketing GM crops, medicines, etc. Mostly R & D firms may hire you if you’re good at using BT techniques.

Environmental science related start ups usually work around CSR clients, but again I did not spot any opportunities in a couple of companies I looked up, easier if you’re experienced in management/finances, other corporate related fields.

Environmental sciences/studies is so diverse, you can work on a hundred different things, from rural development & urban planning to preparing Environmental Management Plan under EIA to working on wildlife conservation.

At last there’s the field of research and academics, and having a good command over your subject(either Environmental Science or Biotechnology) can help you become a Professor.

It should depend on your interests and that would determine how far you progress in your chosen field.

Hope you find it helpful

let me know in the comment box if you are having any further doubts

thanks

Good Luck!