On a related note, does the armour rating of the shield, and whether it is light or heavy, have any noticeable difference?
Just that I am a light armour user, but wanting to make use of spellbreaker, which is classed heavy.
I wish shields were not tied to heavy or light, just shields.
Unless there is a huge difference in armor rating, then no.
As for you second question. By all means, use Spellbreaker. It is the best shield in the game. It will have adequate armor rating, even with a low armor skill (the block skill is more important), and doesn't affect matching set/custom fit.