As most people have already said, it is cheaper, allows more flexibility and you can work at the same time! One of my friends and even my mum are doing an open university course and they love it because they can work at the same time. However in physical uni's you always have support tutors and counselling available to you. Depending on the mental health issues you have, sometimes seeing people in person can help you feel less isolated ... or the whole uni experience (as it did for me my first two years) can make you feel more isolated. Ultimately, only you can decide what you can and cannot handle and both options are great options if you want to do a degree. Good luck making your decision and I hope your mental health improves in future.