I've surfed both the ghost and synergy.
They're completely different boards, for sure. The Ghost really likes to draw longer lines, while the Synergy is a classic HPSB that will fit into much tighter spots. The Ghost will paddle and catch waves much better on a similarly sized board but won't be nearly as maneuverable on the vertical axis as the Synergy.
Right now, the Synergy is the most high-performance board I have in my quiver. I've really enjoyed riding it in 4 - 8 foot waves at OB when it's fun and round but not overly powerful. I tend to ride it almost exclusively on the north end of the beach, where you paddle a little bit less and the waves are a bit rounder.
If the period starts to creep up, I'd definitely prefer to ride a ghost. I don't currently have one in my quiver... I've been riding happy travelers for the last two seasons. I'll probably get another season out of those, but I will probably go back to the Ghost.
I've also surfed the I72 as a round tail and I think it's an amazing combination. I used my I72 round tail when it was fun and 2 - 6 feet, mostly in the fall. I've ridden it in bigger waves as well, but it starts to lose its composure over 6 feet; it's just too fast.