Great question, Eric.
Answer: both, as the majority of breeding eastern and midwestern USA wood-warblers migrate north over the Gulf of Mexico (and, hence, are often referred to as "Trans-Gulf Migrants."
A couple of land-migrating only wood-warblers that avoid the 500-mile over-water excursion include Nashville Warbler and Mourning Warbler.
A qualification: western USA breeding wood-warbler species may travel different flyway routes that never require them to travel over large bodies of water such as the Gulf.
(ABOVE: Blackpoll Warbler autumn migration route (right-most arrows) and spring migration routes (arrows shown in middle of map. Non-breeding range = blue color in S. America; breeding territory range = orange color.)
More on this subject shall appear as an upcoming new post here soon. Please check back.
A fine, general overview of this migration phenomena is accessed at:
A more refined, scientific treatise on this subject is available at:
Plus, here's a species-by-species map of wood-warbler migration tendencies: