Lots of good advice here. Also consider who else might be using your self defense weapon. While I have a 20 gauge shotgun, it's not going to gain much favor with some females!
I like the revolver suggestions. Just like a standard shift car, everyone should be familiar with how a revolver functions. As an added benefit, you won't have worry about reseating a magazine or clearing jabs at 2:00 am with a revolver. Great night stand gun as well as a trainer - and as others have stated, you're hooked now, and next you will be teaching others to shoot!
I own more S&W than any other brand, but if I was going to look at a second gun, it would be something I could use to backup my Shield. Of course, there are undoubtedly a number of us here that have a second Shield (you KNOW it true!) as a backup. That being said, I would consider a Glock 19. Very accurate, easy to shoot, and once you get comfortable carrying the Shield, you will be able to carry the 19 with ease.
As an added bonus, my over 50 eyes like the TruGlo TFO sights, and they make them for both of these fine pistols.