based on your usage, get a 20" folder (pocket *, swift, dahon *). less ride compromise, and still flexible for carrying in car trunks and hauling onto a train (though not into a crowded city bus). a crowded subway, once a week is doable. more often crowded subway rides than that, and you will get annoyed at having to navigate lots of people with a larger, awkward shaped, heavy object.

i had a brompton for a year, but i didn't ride crowded subways enough to justify its small fold size.