Mostly aesthetic differences, but there can be a pretty large difference in how efficiently the mech mod conducts and electrical current.
For instance, your Magneto is fairly average as far as conductivity goes (assuming it's stainless steel). The voltage drop is about .15v through the body which means that with a fully charged battery with 4.2v you're only going to get 4.05v to vape with, the rest will be lost in the body of the mod.
A Nemesis is one of the most conductive stainless steel mods, losing only abut .03v under load. So the same 4.2v battery will give the Nemesis user 4.17v to vape with compared to your 4.05v. This can translate into major changes in available wattage when you're subohming.
For instance, compare the Magneto with the Nemesis using the same 4.2v battery with a 0.3ohm coil.
Magneto = 4.05 * 4.05 / 0.3 = 54.675 watts
Nemesis = 4.17 * 4.17 / 0.3 = 57.963 watts
That's nearly a 3.3 watt difference in power, all because of the body of the mod.
But, like I said at the start, for the most part it's simply an aesthetic preference. The difference in conductivity is only important if you're running really, really low resistance subohm coils.