I'd stick with Magnatiles and get more. My family was used to the Magnatiles, so when I introduced the Magformers, they got a little play, then back to the Magnatiles, just as another poster experienced. I am sure they would prefer the Magformers if we ha started with those, but we didn't.
The Magformers are definitely a stronger magnet, so they can make more complex objects and move them - we had the Ferris Wheel kit, which is a shape that would have crumbled apart with the Magnatiles. But the stronger magnets made them more difficult for my 2-3 year olds to use, because the force of the magnets kept pulling them together before their clumsy fingers could get them set up how they wanted. The older kids liked the complex objects, but still returned to Magnatiles as their preferred tile. And we can NEVER have too many of those. The more we get, the larger their structures.