Here are the basics:

1) The Z5 is the lower end model. Usually black / blue but some come in weather shield. They are very good shooters but have the basic stock. It seems a little hollow.

2) The x7 has a shorter barrel and is more of a Carbine. The ones ive seen wear a nice brown laminated stock. Many people find these very attractive.

3) The standard Omega comes in a variety of stocks and is offered in stainless. They are awesome rifles in my humble opinion.

All have the drop breech which is a great design. They are not too bad to clean and have a reputation as great sabot shooters.

I personally have the Z5 model and it is an excellent gun. I'm sure others can add to this post.