Χαιρετισμούς is more common than Χαιρετίσματα.
We don't often use them on their own. You'd prefer to say "Χαιρετίσματα από τον Γιάννη", "Χαιρετίσματα από την Αθήνα", "Αγαπητέ Γιώργο, θερμούς χαιρετισμούς".
It is perhaps even more common to use "Χαίρετε". I actually often use the singular "Χαίρε" when writing to friends.