The first patio set cover I bought was a thin plastic sheet that fit nothing, billowed like a parachute every time the wind picked up, and tore along the seam by the second month. What it taught me is that a cover is only as good as its fit. Loose fabric flapping against metal or wicker will wear a hole faster than any amount of sun or rain, so the whole point gets lost if the thing doesn't actually sit close to what it's protecting.
A cover is the wrong choice if you're trying to protect something through a real winter in a place that gets heavy snow, since a foot of wet snow sitting on top will pool, stretch the fabric and eventually soak through anyway. It's also not going to save furniture that's already cracked or rusting, it just slows down what's already started. And if your patio set lives under a solid roof or deep eave, you probably don't need one at all, you're just adding a chore. Where it does real work is on furniture sitting in open weather, taking direct rain, sun and the occasional surprise hailstorm.
On color, go plain and go dark. Charcoal, dark olive, taupe, anything in that family hides dirt, pollen and the general grime that collects outside without looking grubby after two weeks. Bright white looks sharp for about a month and then shows every water mark and bit of tree sap, I learned that the slow way. Patterned covers are mostly pointless since you don't see them once they're zipped over a table and six chairs.
The difference between a cover worth keeping and one you'll replace by next spring comes down to the fabric and the stitching. Polyester with a tight weave and a proper waterproof or water resistant coating holds up far better than vinyl, which cracks in cold and gets brittle with sun exposure over time. Look for taped or heat sealed seams rather than just stitched ones, since needle holes let water in no matter how good the fabric is. Vents matter too, a cover with no airflow traps moisture underneath and you end up with mildew on furniture that was otherwise fine. Drawstrings or clips at the bottom keep it anchored in wind instead of sailing off into a neighbor's yard, which has happened to more than one cover of mine.
Measure your furniture with it fully assembled, including any cushions left in place, then add a few inches to the cover dimensions rather than rounding down.