I have played around with that 70-300mm. For your upcoming trip, I wouldn't trust that build quality for such a photo-oriented excursion. The 100-400 is built for Pro use.

I have the new 70-300 IS lens and love it. However, Loupey is right. Since there is a glitch/defect that Canon has promised a resolution for at the end of June, I wouldn't get this lens now. The "glitch/defect" has to do with the build so I wouldn't trust it. The quality of the images is awesome, but for a trip like yours, forget it.

Go for the 100-400mm.

I think Loupey has some great points and I would take his suggestions seriously. He knows what he's talking about.

Liz