I'm breaking my lease at a KC apartment building i've lived in for about 7 years.
in my original lease, the early termination penalty is listed at about $700 to cover advertising and other management fees.
When i informed the building management of my intent to break my lease, they told me the penalty was two months' rent ($1,600 +).
every year, i've signed rental renewal agreements that reflect increases in my rent. but i've never signed anything stating a change in the early termination terms.
legally, which figure should i by required to pay?

It depends on the language of the early termination provision in your lease. Does it specifically say its $700 - or does it say its two months worth of rent?