Hi EstanisgM and welcome to the forum.
It would be very difficult to get an objective recommendation on these brands and models. Considering the prices I doubt there are many companies using several different ones in production.
Additionally, the western brands are notoriously secretive about the origin of the components they use. I have some experience with the Asian options. I am not an expert but I think I can give you some basic information.
There are 2 types of fiber laser sources - MOPA and Q-Switched. MOPA provides a better control over the laser beam but it is slightly less efficient per watt level. Most machines for jewelry use Q-switched lasers.
The size of the work area depends on the optics. Some machines allow you to change lenses for different applications. In general smaller work area means that the laser is more focused and you can get finer detail and better efficiency - a 20W may act as a 40W laser if it is focused on a twice smaller area.
The depth of cutting would depend on the material, lens, and wattage. Gold is easier to cut than silver for example. You can do it on several passes with low wattage or at once with a more powerful laser. 1.5mm is ambitious. Also, the cuts are not going to be straight.
It would be more economical if you get 2 machines - a fiber laser machine for engraving and a CNC Mill (like your old machine) for cutting thick sheets.
As for the versatility of the machine - that would primarily depend on the software.
I am curious what are the prices of the Sisma EASY line?