Magformers and Magna-tiles are extremely popular building toys. They are designed to hold a child's interest and attention, build critical developmental skills and promote imaginative play and creativity. It is difficult to say which is better as each has its own advantages.
Magformers have stronger magnets and work better for 3D geometrical shapes. It is cool to lay out the pieces in a pattern on the floor and then slowly pick up the center one to see the whole thing "grow" into a 3-dimensional object. If you want to make cool geometric shapes, the Magformers are your better bet. Make sure you get enough pieces (normally would recommend getting around 100 pieces to build more complex shapes).
Magna-tiles are more pricey. They have slightly weaker magnets but they are better for 2D arrangements such as airplane runways/racetracks.tec. They can also be used for 3D building such as castles, mazes, but may not hold up as firmly as with the magformers. The transluscent set looks really pretty. Again, make sure you have enough pieces (100+) to not feel held back by the lack of pieces.
There is another option "Magnetic Stick N Stack" with a lower price but weaker magnets. It may be worth exploring as well.