What is worse for the teeth, sippy cup or bottle?

Depends: All things considered it really depends on the liquid inside. If you use water no issues at all, if you use juice both are a problem.

Both can be bad: Do not let your child go to sleep with any liquid other than water, otherwise they may develop many cavities on their baby teeth.

Depends on definitin: As a general rule, the bottle is worse, because sucking on the bottle keeps much of the feed trapped against the front teeth in a vacuum.This allows more access to food by germs that produce an acid that creates cavities. Sip cups with a soft snoot, essentially do the same thing, while hard nosed sip cups allow the feed to drop over the tongue and be swallowed,lessening germ exposure.