I'm responding very briefly to say, it really, really just depends on the individual school. Charter schools, especially, make for an incredibly broad category: They can be progressive community-based institutions, they can be be part of a cookie-cutter model run by a for-profit corporation, they can for all intents and purposes be religious institutions -- anything. With magnet schools, they often have a particular focus or theme -- science and technology, fine arts, etc. It really depends on the individual school and the fit (or lack thereof) for your individual child.
My son does attend a charter school, but it's part of the early model of charters (community-based, Montessori-esque, etc.); since for-profits entered the charter market, this type of charter has become a minority. I chose it because my son is a very advanced reader, and our local public school doesn't offer differentiated education until 2nd grade. I do like it, and I have a world of admiration for the people who made it happen and hold it together, but it's not everything I hoped it would be in terms of gifted education. That's our one individual situation, in one specific area, though. Only people in your area can fill you in on what's available for you.