I would look at which university is best known for mechanical engineering and how the job prospects are ( which has the highest percentage of students going in to employment Use discover uni to find this out ) .
Think about which university you would prefer as you'll be going to that uni. So look at the lifestyle there, the campus itself the modules, the teaching styles and assignment methods as they can make a big difference on your uni experience and grades. Look at which university would help you secure a job at better companies as they'll most likely pay more which will affect how you live your life in the future and look at the general location no one wants to live somewhere they hate.
I wouldn't worry too much about which uni your bf is going because if the relationship is stronger enough you'll both be able to make it work ,think more about yourself and your future.
You can always video call or arrange to meet up half way or he goes to you and you go to him when you both have time off. If he truly loves you he'll support your decision no matter what. At the end of the day you don't want any regrets.