No, the difference is very, very small. I think a meager little "innocent" (in that case: not so innocent, because I hate everyone in Divine Order rags now after my first playthrough) kill in Arx will net more XP than 8 -> 9.
However:
a) Alexandar battle is insanely hard @ level 7 (if you don't cheese him with deathfog like I usually do)
b) Ship battle is much easier @ 9
c) You can start Driftwood with much better gear if you at least reach level 9 because of the vendors
