The design of the Mini CNC is a router, and the new one (which we seem to be calling the Sieg for now), is a mill. Routers generally have lots of x and y travel (like the big CNC), but limited z (up / down), which means they are good for flat objects like panels, signs etc. Mills have a lot more travel in z, so can work on a wider variety of shapes.
Why this matters is just down to what you are working on and how it is held in place. Milling metals, even aluminium, requires more downward holding force than can be provided by the clamping system in the mini CNC IMO. I know members have cut aluminium on the mini CNC, myself included, but I had to get pretty creative with how the stock was held in place and even then I was never able to do it accurately over multiple operations (flipping a part for example).
A vice is a good work holding solution, but the moment you add even a small vice to the Mini CNC + the length of a tool, you lose the small amount of z travel available. Even worse when you need a set of parallels on the vice.
Other differences:
Variety of tooling. The Mini CNC is limited to things that fit in its collet, which is max 6mm. I think I have a ER 16 12mm collet for the Sieg, but anything that fits an MT2 taper will fit in the machine, so you could use a fly cutter / face mill which is good for, well, facing / making big areas flat. Boring bar.
The spindles are different. The mini cnc’s spindle is fast but has low torque (good for wood). The Sieg’s is slower with a max RPM of 6k, but it will perform better at slower speeds than the Mini CNC, which makes it more suitable for operations like drilling, face milling and like some have pointed out, cutting steel.
Just a word on cutting steel, it’s a difficult material to mill. There are all sorts of problems that have to be worked around and it requires a fair bit of knowledge I’d say. If you get it wrong you pretty much instantly break something, be it the tool, your part or the machine (spindle bearings). The swarf / chips that it leaves behind is awful to clear up.
There is probably more, but this is already the longest post I’ve made on discourse. Metal / cnc techs can fill in anything I’ve missed :).