Which Tana IV is better?

Keep your old Tana.
Atk is something that she really doesn't want to lose.
Besides, her hp+ is a superboon (+4) so that workes well with her Vidofnir's def boost.

Due to her defensive lance, +speed would be better in my opinion. With Guidance, your Tana would be good at utility by transporting the strong units and defending others with a defensive build with good speed.

tana is more of a tank/support units with nuking capabilities so her atk isn't all that important unless you run a glass cannon build