What's better out of the Dexter books and TV series? Same can be asked of True Blood. If the answer's the books I'm gonna hop right to it.
For Dexter, they both have their own qualities. Perhaps the strongest aspect is that, without spoiling it, they are very different from the show storywise. The characters are the same, but don't be put off reading for suspecting you've already seen what you're gonna read because it's not the case.
IMO, the TV show has been excellent upto the end of the fourth season, and the first few Dexter books were great and really original I thought, but, I don't know, maybe too much is a bad thing.
Let me explain quite quickly, when I read the first two books one after another I was desperate for a third, then desperate for the fourth and by the time I'd read it I wasn't excited for the latest fifth book. I read it, out of continuity, and it's a fun read but it's not what I hoped. This is all personal to me of course, I've read reviews online and people love all the books especially the latest so what do I know? lol
I definitely recommend them at your own leisure, without worrying about the TV show as nothing past the first book is the same. Some are drastically different. They are very, very quick reads and it's never taken me longer than a couple of days to read any.
If you like the TV show, read the books definitely.