Both are great Valks and the 25 is a very cool thing especially Ozma's which is in my mind the pick of the Messiah bunch but personally I would be getting the VF-19 Advance.
Its a fresh newly designed and technically more advanced toy and even though it will probably cost you a little more than some examples of Ozma which are available out there but in terms of value for money I definitely think the 19 is the better buy! And I found it a far more enjoyable toy to handle and play with compared to a 25..