Milling Steel?

[i2i]

if you go with a small diameter carbide or coated hss slot drill/end mill, you're going to cope better with the higher speeds that the router gives. Then create a localised coolant dam to keep things cool, you'll probably get by.

[JAZZCNC]

A mill having a fixed Z turret and moving X Y slides, and a router having a fixed table and moving gantry?

So if wanted to mill, say chocolate moulds, , as an example thats come up before,fairly deep/ high level of finish, in steel, would really need to be on heavy iron fixed turret mill to get there?

If the routers frame and Z axis is strong and you keep the extension to a minimum by lifting the work to the spindle then it will do it no problem with the correct end mills. Yes Slower when it comes to removing large amounts of material but not when it comes to finish on small things like moulds.
The moving bed or gantry doesn't really come into it. Just more common for mills and easier to build strong machines with limited travel if fixed gantry.

The higher speed of router spindles will actually give you a better finish than slow milling spindles when using small end mills needed for detailed moulds like you suggest has they often don't spin much above 3000rpm and 10,000 rpm + is not uncommon.

That said if I was mainly or even just more than occasionally cutting steel then I'd buy a Mill with the fastest spindle I could afford or buy speed increaser.!! Steel really does stress a router out.!

[m_c]

A mill having a fixed Z turret and moving X Y slides, and a router having a fixed table and moving gantry?

Quite a few CNC mills have fixed tables, it's just that conventional mills were all pretty much moving table.
The difference between a mill / router is more of construction than anything else, with mills generally having lots more weight with a lower speed spindle capable of high torque, whereas routers are generally lighter with a higher speed spindle/less torque. There are off course lots of exceptions to those, with some routers being more than capable of handling steel, and mills with high speed spindles, but the big difference is the mill hold tight tolerances all day long, whereas routers are generally not as accurate.