Haven't been to the Snaefellnes Peninsula, so can't help with that.

Have been to the South Shore. Unfortunately, you don't say which place(s) the tour you are looking at will visit. The South Shore tour I'd taken was worthwhile. The tour visited some scenic sights near Vik (black sand beach, rock formations), waterfalls and glaciers (which you will see in the Golden Circle). It is a long day, just be prepared for that.

I'd used Reykjavik Excursions for my day tours and I was happy their service.