Cover and structure are not necessarily the same thing. Cover is basically anything that can hide a fish or baitfish, whereas structure is basically anything that causes a change in the bottom contour. Hydrilla, lily pads, cattails, and trees are examples of types of cover. Points, humps, creek channels, bridges, and pond dams are examples of structures.
Which is best? It all depends on where you are fishing, the time of the year, and how you like to fish. I think most people would agree that a combination of structure and cover is best most of the time. In fact, I think most people fish both without realizing it. If you are a bank beater throwing at visible objects, you are fishing both - the objects are cover and the slope of the bank is the structure. In many instances, that can be a good strategy. However, on pressured lakes those visible objects get pounded by fishermen every day making them less and less productive.
As a guide, I tend to fish structures that aren't visible to the eye, except by using electronics. Some of my favorite structures are humps, points, pond dams, and road beds. You can find many structures with the aid of a map and electronics. But so can all the other fishermen. I prefer the ones that can't be found on a map. However, they are few and far between. I also prefer structures that have some green vegetation close by. Plants come and go and change with the water level. So you never know when a spot will have greenery. You just have to put in your time or pay to find those kind of spots.
When the lakes are low, I keep an eye out for structures that will be productive when the lake fills back up. I especially take note of hydrilla lines that extend several feet away from the bank, knowing that when the lake fills up the average fisherman won't hit that spot. Thick hydrilla can be a combination of cover and structure by itself, providing cover for baitfish and contour changes for ambush points.