Nana is pretty- because of all them curves, pretty face etc. But Tachi is freaking sexy. Way more sexy than Nana
If i were to compare them... In "Sherlock Holmes"(2009) there are two actresses- one plays Irene Adler, and is prettier, but other, who plays future- Mrs.- Watson is freaking hot as hell...
Now, what your preference is, that is up to you, but while i always enjoyed Nana being toyed with, i also get really nasty imuplses watching Tachi trying to even the field, by biting back. While Nana is submissive, and takes whatever her Master throws at her, Tachi`s squirming and pretending to be fighting back is also awesome... I really had nasty grin, and evil feeling stirring up inside me, while i was reading that chapter, watching her being unable to run away. Last time i had similar thing happen to me was waaay back, when in first manga Nana tried to sneek a peek at Tachi breather and got caught, and Karou was telling her how much of a bad girl she was...
... I just realized i am now probably on FBI "Possible future rapist" watch list as of now... As if i weren`t on "Possible future serial killer" and "Possible future religious fanatic" already...