I watched the first few episodes of "The Strain" and, to be honest, found it to be needlessly brutal. It just didn't engage me. The actors were fine and the writing was above average, for television fare. It's just not my thing. Plenty of my friends really enjoy it and I would suggest taking their recommendation over mine. It's worth watching, at least to see if the story will pull you in as it didn't me.
"The 100" is something that I've watched more than a few episodes of, though it's rather predictable. As with most of the shows from The CW (network) it involves improbably pretty people; in this people in their late teens, who are somehow more wise than their elders. At least some of them. If the idea of "Lord of the Flies" as played out by supermodels is the sort of thing that intrigues you, you might enjoy it. Where I really wanted to like "The Strain" and just couldn't stick with it, "The 100" is the sort of train wreck that I have difficulty looking away from.