I believe I would rate the "High Voltage" spec the best one at this point now after doing more real world testing on them both. Yes you can solo a golem with the merc spec. However, helping your group with the HV spec allows them to better take control of the game. Your team if they are doing well will take the golem anyway and its usually more important that you help them with objectives, lane push, and other group fight scenarios.

Something I find interesting is the amount of damage the spec does. It seems like it pulls ahead in siege, hero, and xp contribtion. Which is another reason to pick it over the merc spec unless you just want to solo a golem.