Nubbins!
The nanite mesh generator is a shield for the heavy assault, how is it different from the standard?!?! No idea, but you shouldn't be using it, you should be using the Resit shield. this is your classes special ability its a different tab to the suit modifications.
for suit modifications there several shields, some enhance that shield effectiveness or how fast it will begin regening, other options just allow you to carry more ammo but with the default shield. The advanced sheild capacitor full certed will begin regenning in about 8 seconds where the default takes about 15seconds before it starts. they all take 4 seconds to regen from 0-full(could be like 3.5 seconds)
you can see the certs here:
http://wiki.planetside-universe.com/ps/Certifications
and the different slots they take up. no idea why the resit shield isn't listed, but anyway.