It depends on how long the item will survive in combat. 1500 shields from the 2 shield systems is not as good as the 2900 shields the shield regen system will generate if the ship survives the 29 availible combat turns and is dammaged. Of course if it is undammaged neither system matters.
So my analysis is if the ship should survive more than x turns such that (shields per kt)-(x * regen points per turn per kt)=0 use a regenarator instead.
sorry seem to have geeked out
Also if you can rotate your ships through the battle the regenerator becomes better.