Centrifugal brakes will use the speed of the rotation of the spool to, in turn, slow down the spool. The faster the spool spins, the more the brakes act on it. The spool will be spinning fastest during the initial part of a cast, which (at least for me) is when the big nasty back lashes happen. Magnetic brakes work according to the distance between a magnet and the spool. This system allows for brakes to be applied ALMOST directly proportional to spool speed. This means it will act on the spool, especially towards the end of the cast, but does not act as well during the initial high speeds of the spool during a cast.
If you can find a reel with both, great. There are obviously some nice ones out there. But, between the two, I prefer centrifugal. On a reel like my Lews Tournament that has both, I rarely have the mag brakes on. On most casts I am able to use my thumb at the end if I need to. Obviously this is near impossible to do in the beginning.
I have no experience with the SX, but from my understanding, the problem with the Gen 2 was that it was very difficult to dial in, which led to a lot of backlashes. The new Gen 3 SX sounds as if the problems have been fixed, or at least improved upon. However, for my first casting set up, I would be leery of purchasing a reel with only magnetic brakes. I feel the centrifugal system will be easier to dial in and not present as steep a learning curve. I could be wrong, but that's my 2 cents.