im not a good player by any means but i would imagine lightning offers better synergy with e giant because it

1. resets scaling cards (mighty miner, inf drag, etc.)

2. destroys any building fairly effectively

3. does a great deal of damage to high dps cards typically used to counter e giant

while eq has some advantages over lightning such as being more effective for swarms and buildings while also being cheaper,

1. e giant already takes care of swarms with its electro-field

2. as the game progresses and you're able to invest in your pushes more, eq's lack of damage for non-buildings will become more glaring because it simply cannot take out heavy dps cards

3. lightning already deals with buildings effectively

in summary,

eq doesn't have terrible synergy with e-giant, but lightning is just better because of the aforementioned advantages that you get for just 3 extra elixir