Alice is deciding between two internet service plans. Plan A costs $50 per month with an additional charge of $0.05 per megabyte (MB) of data used. Plan B costs $70 per month with an additional charge of $0.02 per MB of data used. If Alice uses 1000 MB of data per month, which plan is cheaper and by how much? Let's think step by step and output the final answer within \boxed{}.
user