I wouldn't say they are necessarily better. They are helpful for seeding new tanks, but then so is regular biological media like sponges and ceramic rings...personally, I find the biowheels wear out and start to get too gunked up too turn properly, or otherwise get beaten up and misshapen so they don't often make a full rotation. Supposedly they are working no matter how fast they turn, but when all they are doing is turning every once in a great while, I just chuck them at that point.
They used to be cheap, but now you can't find one at an LFS for under $10 here! It's crazy. (I'm talking about for the little ones, like the ones that go on the Penguin 100...yeah.) And online is cheaper of course, but then there's shipping...
Speaking of cheap, there are frequently excellent sales, so never pay full price. Amazon has the best deals consistently, but once in a while Big Al's will have a steal on Marineland filters, especially if they include a rebate coupon.
United Pet Group has overall excellent customer service, by the way. They are a monopoly of sorts in the pet trade...Marineland is one of their brands, along with Tetra.