You want the discharge to be above normal snow level, so that may dictate where it goes. I'd rather cut a hole in the siding than the block. You don't want to use flexible ductwork - while harder to install, you're much better off with solid, smooth walled duct. the clothes will dry quicker and you'll have less (potentially dangerous, fire-prone) lint buildup. I've heard good things about one with a U-shaped outlet with a cap on the end. WHen the dryer is running, it pushes the cap up, it falls back when off. No springs to wear out as gravity always wins.