Well, WoSign's intermediate certificate is signed with SHA1, which is weak. Probably to keep compatibility with outdated Windows XP installs (which are common in China).
Funny enough, their intermediate certificate is signed by StartCom, another CA that offers
free certificates. However, StartCom charges for revocation of their free certificates for any reason, including after HeartBleed. The WooSign website also uses a common DH prime, making any forward secrecy there useless.
They still offer SHA2 certificates and their website setup won't affect your own, so it's not
that bad.
Still, Let's Encrypt is signed with SHA256, and their website's HTTPS is setup a little better. Their biggest advantage, though, is the automated client that takes care of creating a CSR, validating it, and adding it to your Apache/nginx configuration automatically. They're aiming to be "the" free certificate authority.
All in all, Let's Encrypt seems to be the way forward.