What are you going to use the gun for? Either gun will be a good carry weapon. Quality is just opinion, but I love my S&W. The glock will be a straighter shooter for sure because of the heavy trigger of the S&W. But I think the S&W heavy trigger makes for a better carry gun for begginers. I'm like you though I prefer to buy new but will not pass on a great deal. A lot of those used Glocks were used by cops and may have had a ton of rounds through them.

The gun store should let you use the $100 on whichever gun you prefer. Let us know what you decide, and post some pics! Good Luck!!