Telephoto Lens Selection

So i've been giving it thought and I want something longer than 200mm....

I have a 7D and 5d3 and so far i've got three things i've been considering:

Buying a x1.4 or x2 extender (My 70-200 is the 2.8 v II)

buying a 300mm f4 IS

or

buying the 400 5.6 lens.

So i'm trying to consider what would be better, now I shoot everything weddings, events and landscapes...

i'm mainly considering one for events and racing and i'm kind of stuck at the moment between the 300 and the 400..... The 300 is a stop fast AND IS (Which is good for hand holding), but the 400 (While a stop slower) is 100mm longer (And unstabilized). Keep in mind that I'm pretty steady so I don't HAVE to have IS...

What do you guys think?

First get a 1.4x TC, something you'll use a lot later no matter what you wind up getting. Tamron SP would be my suggestion. Use it on your 70-200 and see how that goes. Should give you an idea about needing the 400 or not.