Aren't they basically the same thing? In your example 2., you can say you still distribute $15K to everyone, but you set the progressive tax rate so that if you get $60K the extra tax amount would become $15K that cancels the flat distribution. (If you say UBI is for every person including kids, you can still adjust tax rate in 2. with dependent deductible/compensation).

The amount people get depends on the design of whole scheme (including tax increase to compensate UBI) so you can't say which is cheaper/expensive in general. Can you? (I'm not an expert, so my reasoning may be flawed.)