MW reception is often plauged by man made interference these days. Most man made interference comes from small close sources like say a laptop power supply .Is a loop antenna better than a whip when man made noise in the near field is significant ? Is this near field man made noise predominantly electrostatic ? magnetic? Or is it both like the desired far field signal? Loop antennas do work for MW DX but it there more to it than the ability to directionally null out interference?
I'm going to assume that by "loop" you mean a "magnetic loop" or a "small loop". That is, one with a circumference significantly less than the wavelength. Larger loops, with a circumference approximately equal to the wavelength, are more like folded dipoles and behave differently.
Is a loop antenna better than a whip when man made noise in the near field is significant?
It's impossible to say in the general case. Each will pick up different fields in the near field, so it will depend on what kind of noise is around.
Is this near field man made noise predominantly electrostatic? magnetic?
There's a good mix of each.
Loop antennas do work for MW DX but it there more to it than the ability to directionally null out interference?
Yes and no.
In the very near field, a loop has a lower field impedance than a whip, meaning it's more sensitive to magnetic noise. Farther away, but still in the near field, the loop actually has a higher field impedance than a whip, making it more sensitive to electric field noise. And in the far field the impedances are equal, as required by the laws of physics.
So which is more effective at avoiding near-field noise really depends on local conditions which are hard to predict.
Although loops and whips have identically shaped radiation patterns, those patterns are oriented differently with respect to the antenna's polarization. In each case the free space radiation pattern looks like a doughnut with two nulls. For a vertically polarized whip, the nulls point up and down, but for a vertically polarized loop, the nulls point at the horizon. Of course the presence of ground alters this in a way that depends on the height of the antenna.
So if you must have a vertical polarization (as is the case for ground-wave propagation), then the loop gives you nulls pointed in a more useful direction. This certainly can be part of its effectiveness, but as previously mentioned the differing near field impedances can help as well, depending on what kind of noise is around.