Forceshield is a wide 4+ invulnerable save in the TT but is a personal shield. i.e. The wearer benefits only.

Shimmershield is and advance forcefield that gives a 5+ invunerable save to the wearer + unit in the TT. i.e. The wearer stays alive then the unit gets the benefit.

In EC:

I would expect the Forceshield to be wargear unlocked for the Autarch and Wraithblades and be either a Armour boost or possible a new shield mechanic yet to be implemented. (Armour boost would be the easiest to do)

Shimmershield, I hope, is a Dire Avenger Exarch only wargear upgrade but would give a Armour Boost (or other shield mechanic) to friendlies in a certain range. I would expect something around the "3 meters of the user" mark for this. Would give a nice 6 meter bubble and turn the user into a grieve magnet...

I certainly hope that the wargear for the Exarch's allow appropriate buffs based on Aspect.