Christina is definitely the more classic sounding, although I don't mind Christiana (I like Christabel too, but that might be a bit more "out there").
Christina is nice - some people may say it's a little dated but I think that's more Christine. Christina reminds me of poet Christina Rosetti personally and like the variety of nn's (apart from more dated Chris/Chrissie/Tina, there is also Kit, Kitty and Christy which are nicer imo).