Although I like both names,
Tatiana wins.
Tatiana is beautiful, romantic, and has several nice nick names:
Tiana,
Tia,
Ana, etc.
Tatiana makes me think of a queen or a fairy godmother.
Serena is lovely, but the 's' is sort of slippery.
Rena is a nice enough nick name, but it doesn't really do a whole lot for me.