Depends...
On the DAC (damage allocation table), there's only 2 kinds of hull hits, forward and rear. Center hull gets knocked out once all forward or rear hull is gone.
With the 18 / 6 / 6 combo, you could get up to 12 "rear" hull hits, as the first 6 RH hits would wipe out the rear hull then the next 6 would wipe out center hull. On the other hand, that same combo could get you 24 FH hits, the 18 forward then the 6 center. The thing is that both FH and RH tend to get hit almost equally (over time, there could be bunches
Also, in the DAC, after FH is gone, weapons tend to be the next things destroyed, while engines tend to fade after RH is gone. So, the 22 / 0 / 8 combo is very protective of weapons, but lets engines die quicker, while the 18 / 6 / 6 combo tends to protect a bit better, but still favors weapons slightly over engines.