I dunno wich to bye! I'm not rich, therefor
I need to know, at least, the main difference
between these products.
Can anyone help?

What I would do is get the money saved up, and then go to a big store like Guitar Center or whatever you got in your neighborhood. Ask if they have a satisfaction gaurantee, make sure it's 20 or more days, and just buy one and try to get as deep into it as possible.
I've owned an X-3 for about 8 years now and love it, it's a music workstation. The Trinity (1997 released)and Triton (1999)are music workstations. That means they have an on board sequencer so you can make a song right there using the keyboards sounds. I'm looking at getting a Trinity because it is so damn upgradeable, and the sounds work very well for dance music.
The Zi is a synthesizer. What it is for is live performance and sound creating and editing. That means you need a separate sequencer (or some other device) to record a song onto.
I should add that the new Trinity v3 has all the features (i could be wrong) of the Z1, and an older Trinity can be upgraded for about $600.
I hope that helped.