~세요 is more formal expression. ~요 is also formal too.

They are both formal but the only difference is we use 있으세요 to people who's way older than you or if you have met a new person (usually first time). If someone is only few years older than you just use 있어요 :)

있어요 - formal expression 있으세요 - an honorific title more than 있어요

~세요 is a markedly more respectful version of ~요.