They are very different cities. Swansea is what you would call a proper city. Reasonably larger with a population of 1/4 million. Bangor although a city is more like a town. It has a population less than 1/10 of Swansea's. It really depends on what you want.
You dont have to make a decision until June. The Welsh Government has opened up travel throughout Wales from tomorrow to people living in Wales and to people from England from 12 April if the England lockdown is removed. Hotels are opening in Wales from tomorrow. Both universities are likely to reopen on that date. It is likely they will be holding visit days for office holders from May. Even if they dont you can visit each university and city to see which is right for you.