There are some series it's just not worth it to read the manga plus watch the anime, they're just too similar
to each other.
If I really like a manga I'll watch some of the anime to decide if I want to watch it in addition to reading the
manga. And if I really like an anime I'll try the manga, especially if I know the manga continues on past the anime
or is recommended as being better then the anime.
But usually I'll stick to either watching or reading a series, not do both. Some exceptions are Fruits Basket,
Pretear, Fushigi Yuugi and Card Captor Sakura.
Pretear and CCS because the manga + anime are different enough to keep things interesting, Fruits Basket because I just
really love that series, and FY because I know the manga keeps going after the tv series ends and I like FY enough that
I don't want to miss that part. (as far as I've read the manga follows the anime pretty closely, oh
well)
Prince of Tennis I started with the manga and then switched to the anime - I like how the anime spends more time with
the other Seigaku players besides Ryoma.