I don't think there is a 'better' between the M!X and the P's, as the gameplay is so different. It's really more personal preference. I'd recommend perhaps watching some videos on YouTube (if you haven't done so already) and seeing which style suits your fancy. That said, I'll list a couple of pros/cons for each model.
P's
Pros: Can be patched into English (although as your first language is Spanish, I'm not sure if this is really that important for you?), lots of downloadable content, lots of inbuilt content/lots to unlock
Cons: Can be expensive, pierces can be expensive, requires three P's too unlock every inbuilt character
M!X
Pros: Cheaper to buy, 'bigger' screen (no icon bars) so more detailed graphics and bigger sprites, more replay value, can unlock everything on just one unit (Anniversary version only), really cute animations
Cons: Only in Japanese, no downloadable content, less 'inbuilt' content, requires at least two units to unlock all the towns (for the Melody and Spacey versions)
Let us know what you decide! =)