Having worked for Sears for a number of years whule attending college, I can add some insight to the discussion.
The problem really comes back to the "1 1/2HP" as stated by the OP. Some time ago, the power tool and appliance industries caught on to the fact that "HP" is not a legally defining term in that it can mean enegy consumed or power delivered. They took that at it's most liberal inrterpretation and began using the marketing term "develops" in front of the HP on the literature. The truth is in the electrical ratings label. If the label says the motor draws 18A, that is the Full Load Amps of the motor. By taking advantage of the breakdown torque capabilities of the motor, they can make the "develops 1.5 HP" statement mean that at some popint in the torque/speed curve, the motor shaft HP may be as high as 1.5 HP. On a continuous basis however it is far less, and of course you are not typically using a saw on a continuous basis.
That said, the 18A motor would theoretically be a smidgin more powerful, and that mey translate into better performance but all in all the difference is probably slight.
A distinct possibility also exists that this is just a semantics issue as well. You didnt state the voltage, but lets assume 115V nominal since that fits at least close to the current rating. If one of them has a nameplate voltage of 110V, 18A and the other says 120V 15A, they could be referring to roughly the same power capabilities.
In summary, the nameplate ratings will have little information from which you can judge quality, as others have said earlier. I just win the award for excessive elucidation.