Both Testaments equally display God's holiness/justice/grace/mercy. They're all the same thing. His mercy isn't the opposite of his justice; they're the same thing. Jesus, likewise, talked a great deal about judgment... so it's simply inaccurate to say that God shows more of (or, only) his "just and holy" side in the OT than he does in the NT, or vice versa with his "grace and mercy"... they're equally displayed in both Testaments.
And the dichotomy really shouldn't exist to begin with. Separating the Old Testament from the New Testament like we're doing here is like separating Genesis from Exodus, or 1 Kings from 2 Kings, or Romans from Galatians. You might find some emphasis on certain things between each individual book, but that doesn't mean the other things aren't there at all.