I have a 3w 180w panel and it is awesome so far. Still vegging first crop with it, only 2 plants but they are loving life under it. The more watts you go with the greater coverage area you'll get. Having never used the ufo type models, im guessing you can get more creative with the placement of it, especially if you decide to use more than one because they are smaller in size. My recommendation is do lots of independent research on the companies, choose one with good quality lights (cree, bridgelux, or similar), find one with a good warranty, and make sure you can call and actually speak to someone at the company. I totally agree with ChroniCDooM about spending a little extra to get a good quality light.
You can use any type of medium you wish, but know that the led lights give off very little heat so whatever medium you choose will not dry out fast at all. personally I use coco and the plants are doing great under the combination of led light and coco. Check out my journal if you get a chance.