It's easy for you to decide between those 2 choices. First off what are you better at creating? Modern structures that have a lot of structural design, or are you better at making medieval looking buildings? If you think you're equally good at making both of those kind of themes, then really it's a matter of preference of which you like more. Do you like the feeling of a RPG server, or that of a booming city with industries. Now if you still can't decide then I guess your last option is to go on a list randomizer website and put your two options there and see what the outcome is.