My Motorola Triumph was $300 USD when it first hit Virgin Mobile. Now it is $279 USD, IIRC. Even assuming a $100 difference in price, the difference is made up in 3 or 4 months max considering I too was grandfathered in with the $25/mo unlimited sms, data, and 300 min plan. And as others have said, service is comparable.
Definitely the way to go, unless you absolutely need to rely on a data connection (work purposes, etc). VM's is sufficient but certainly less than desireable.