I have been bouncing around the internut and am now cornfused.  I need a little more of a comprehensive comparison.

What are the main differences between the old gen (raised lower rec metal in front of the trigger area) or latest (smooth)?
If I wanted an older gen, what would I stay away from or look for?

You want to find an early JS series 556 Gill Guard models, or one of the Swiss ones with 4 screw top rail, they have the most Swiss parts in them. The later models Quality control started slipping, and you can see how they started short cutting the lower receivers by making them smooth instead of ribbed.

True...there were quality problems for a short while as SIG changed over to the new style receiver, but everything since has been fine. Both my 556 and my 551A1 are two of the finest rifles you can imagine. If you want a good AR alternative these are the rifles I recommend, a joy to shoot and maintain. With the SIG red dot sight, even though it looks cheap, in fact works really well, is very light, and completely co-witnesses with the irons. The rifles shoot anything you put in them, and mine are 100% reliable. I also have a 516 which might be the best AR type rifle I have ever owned. I have shot the Swiss rifles and these American versions are very close in function...in cosmetics the Swiss rule...but if you lay out the internals and check them side by side the U.S. parts are near identical and interchangeable. The 556/551 is one if the most underrated rifles in the U.S. market. and getting harder to get.