I guess it depends on the date you want to portray but, since this is VSF, there is no reason why you shouldn't mix the two types of troops. For Spanish-American War period uniforms the following manufacturers are worth looking at:
Redoubt:

Redoubt's Boxer Rebellion range has some very nice US marines. These are very nice 28mm sculpts in slouch hats and shirt sleeve order. The range includes a couple of options of Colt MG on wheeled carriage.
Tiger miniatures:

The Tiger US troops include marines, infantry, mounted cavalry, dismounted negro cavalry and artillery. Most are in tunic and slouch hat (except the marines who are in peaked cap). They are slightly taller than Redoubt and and their rifles are obviously larger. The marines packs include Gatlings and Colts on a mix of wheeled, tripod and ship mounts. There is also a very nice marine landing gun in this range.
Parroom Station (London Warroom VSF figures):

'Slim 28mm' figures of US infantry in slouch hats and shirt sleeve order.
I am collecting Americans to make a US marine force suitable for Boxer Rebellion or Spanish-American War, using Redoubt for the core troops with additional figures from Tiger and Parroom Station to help make the force look more non-uniform and provide additional command and support troops.