Highly recommend Aria of Sorrow rather than Dawn.
The art style in Aria holds its gothic scenery while Dawn goes for an anime-style, which I highly dislike for Castlevania games because the gothic art is far more beautiful and detailed than anime in everyway.
Aria adds a lot of things to the series and is quite addicting actually, completed it with all souls, reached level 99 and 100% completion.
Definetely the best Castlevania game in the GBA by a mile.
If you are going to get a Castlevania game for the DS, I recommend Order of Ecclesia instead.
Just my opinion though, hope you find it useful.