Almost none, and it is worth being straight about that before you spend a weekend searching. Dubai Marina was master planned as a high density tower district around an artificial canal. Land there is too valuable and the plots too narrow for villas, so what you find advertised as a "Dubai Marina villa" is usually one of three things.

What the listings actually are

A villa in a neighbouring community, marketed under the Marina name because that is what people search. Al Sufouh, Emirates Hills and the Jumeirah communities all sit within a short drive.
A townhouse or duplex inside a tower. Some Marina buildings have ground floor or podium level units with a terrace and their own entrance. They are genuinely different from a standard apartment, but they are not detached houses.
A penthouse. Large, expensive, and nothing like a villa in the way that matters, which is having your own land.

Where to look instead

If you want a villa and you want to be near the Marina, these are the honest options, roughly in order of distance:

Al Sufouh. Low density, coastal, behind Palm Jumeirah. The closest genuine villa stock to the Marina and priced accordingly. See Al Sufouh.
The Meadows, The Springs and The Lakes. Established family communities off Sheikh Zayed Road, ten to fifteen minutes inland. Mature landscaping, which newer communities will not have for a decade. See The Meadows.
Emirates Hills, if the budget is there. Individually designed houses on very large plots. See Emirates Hills.
Jumeirah Park and Jumeirah Islands, both a similar drive and considerably cheaper per square foot than the Marina itself.

If it is the Marina you actually want

Then the question is not villa or apartment, it is which tower and which floor. Aspect matters more in the Marina than almost anywhere in Dubai: two identical layouts can differ sharply in price and in liveability depending on whether they face the water, the canal or the next tower.

Tell us which of the two you are really after, the location or the format, because you generally cannot have both here. Our guide to Dubai townhouses covers the middle ground.