If you want to print on demand then it would probably be vinyl or
dtg. If you wanted to stock x amount of t-shirts then you would probably go for screenprinted. You would need a fair amount of money and space to stock the screened t-shirts as you would need to order about 100 of each design to make it worth doing, however you would only need to make them as they sold if you use vinyl, small set up cost of about £1000 for cutter, vinyl and some t-shirts.