Both are essentially the same pen right ?
The size, nib, styling, trimmings, material used are the same.
The old style M600 can be found seling at a cheaper price than the new M400.
Isn't it a better idea to get it than the new M400 ?

Nib is different, bitone 18K from Pelikan factory, not 14K Bock. Trimming very similar, some more extra rings in the modern m400. Same size.

Is 18k nib better than a 14k nib ?

Not necessarily. I find no difference at all while writing, if any, they are stiffer and less springy than the 14K ones.

ISn't more gold content mean that an alloy is more pliable and less stiff ?
Read somewhere that the old M600 may come with either 14k or 18k nib.
18k nib should be more expensive right ?