Definitively first the HoN/IB. This is the more clever choice.

Why ? No more HoN/IB = no more mines, no more tech/hotwire = no more defenses

And as GDI no more f00king stealth-nuker...

You open the way for easy vehicle attack and easier beaconing.

You can do this too if possible : http://messagebrd.westwood.ea.com/cgi-bin/boards/ren/english/ultimatebb.cgi?ubb=get_topic&f=2&t=000653

A team who lose all but HoN/IB can win. I've seen/done it a lot of time... The opposite is true too : I never seen a team l00sing HoN/IB at the very start of the game, win in the end...

Then destroy factory/airstrip and your enemy becomes harmless. And yes with no tech/hotwire/mines trying to defend against you, that's just easy... Now what's the use of refinery or power plant alone ???? Let them try to nuke you with character at 0 cost (lol), you will disarm all and increase greatly your score !

On defended maps with AGT/Obelisk :
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Yeah depend of the map. But for a real victory, no real choice : power plant or tower is best.