Well, in the world of quality pistols you do tend to get what you pay for
Not that Sig builds a low quality pistol, just that not all Sigs are created equal. Good example is the car world - costs more to buy a Mercedes than to buy that KIA. They both do the same basic job. In 15 years the Mercedes will still do the job comfortably and competently. As to the KIA, not so much.
I will always go with higher quality alloy frame rather than the polymer. The couple of ounces saved on weight isn't that big a deal and I know it is the best that Sig can build. I see the additional cost as peace of mind. JMHO and I know many would disagree, certainly their right and I won't argue the point.