The Kensai gets a much better THAC0 at low level, -2 to AC, bonuses to damage and speed, depending on how many levels you take. KAI can help especially at low levels.
The best way for you is to test a couple of char's with SK and check their stats. You'll probably dual at level 9 if you play SOA only so check the differences between kensai and fighter at that level.
And remember: THAC0 doesn't matter in Time Stop.
