The external mic definitely helps for video, but so does the viewfinder. Holding the camera up to your face gives stability. I'm actually kind of miffed that I can't do both at the same time. Which is why I was thinking of getting the best of both worlds and recording the audio on a separate recording device and then synchronizing with the video. To do that, you have to have good audio from the camera to be able to match to the external audio, and it could be time consuming.
There are software programs that will help with synchronizing.
A tip picked up in another post is to use a sharp noise just before filming starts that is recorded on the internal and external audio. Makes syncing easier. In movies they use a clapper board (although this is probably electronic now).
I tried the external mic. When mounted on the hot shoe, it was worse than the internal mic because it magnified the sound of focussing, even with manual focus of kit lens. When taken off the camera, it was ok but not great and not worth the hassle or the cost, or having to give up the viewfinder. I'll be sending it back and looking into an external recorder and learning how to sync when it's really important.
On the positive side, this makes the choice between the mic and the viewfinder a lot easier!
I tried the viewfinder. I found it not as as useful for video as I hoped. First you can't magnify the image while recording the video. This is whether using the LCD or VF-2. But with the LCD, a magnifying loupe will help a little. Second, you can't switch between the LCD and the VF-2 while recording. This is something I'd want to do while the camera is on a tripod. I still like the VF-2 a lot for photos.