Really depends on what you would like, as a career.
Mental health nurses, have to deal much more with the law than adult nurses, in particular the Mental health act.
Mental health nurses focus very much on crisis situations, where a patient may be suicidual and in some cases carrying out sucicial acts. Your ultimate job is to provide therapy, medication and build a relationship with a patient slowly guiding them back to good mental health. On the other hand the job has its ugly side, such as restraints, seclusion and even forcing medication on patients through injection. Obviously this would only occur if they were very ill and unaware. Nurses very much manage the ward, rather than treating patients although they have a high degree of participation.
Now nurses, can become prescribers, however the role is new and the roles are often difficult to gain although not impossible.
Doctors, on the other hand clearly treat patients, your job is to diagnose the patient. You will view the body ultimately as a machine and seek ways to fix this often by medication. However you have the option to explore various different aspects of medicine and surgery. You wouldn't be limited, to purely psychiatry until you decide your speciality which will come at a much later stage.
You will also have less patient contact and very high responsibility, people's lives will literally be in your hands. The nurse although very experienced will often look to you, to provide the answers (although there are very skilled specialist nurses, where you'll be without a doubt bowing to them).
Medicine 6 years of training vs 3 years for a nurse, medicine is a more prestigious degree however very time consuming. Nursing is also very time consuming and intensive, however you'll have much more of a life being a nurse. Especially until you reach the high ends of medicine although it does depend on your specialty.
If I could guide you on picking either, I would honestly say none. The NHS is in a terrible state. Go become an accountant or something boring but you won't be stressed to death, through having stupid targets you can never meet, as you simply can't do everything. You'll be doing ever hour under the sun as a doctor.