15mm is so much more epic - more troops on the table, and pretty (One-)king-size warmachines and beasts to run said troops over.
Also, I´ve said it before: 15mm are ridiculously cheap. Doesn´t necessarily mean you spend less money, but it means the army you get for the same cash is something huge.
(Actually, using 15mm miniatures such as the Wayfarer range as proxies for Chronopia - while keeping inch distances the same as for the 28mm figures - will result in a much more realistic feel to missile ranges. I´ve tried it with proxies and was very satisfied with the results.)