For the particular area mentioned, which I've spent a bit of time in, IMO a poncho is not the best choice. In August, the trails in New Hampshire, Vermont and Maine will have developed pretty significant brush growth. I use ponchos in other seasons, but it seems late summer I'm constantly catching the poncho on every trailing branch. In addition, there are a number of places you'll be crawling over or under things, which I find to be a huge pain in a poncho.
In addition, there is the Whites. Unless you're far luckier than me, you will have rain, fog, maybe sleet and snow. And the wind will blow. Not a breeze, but a nice sustained 25-30 mph wind with gusts to 40-50 mph. And that's a decent day, it can get far worse than that. I wouldn't be caught dead in the Presidentials with a normal poncho. That said, my preferred choice for backpacking is a version of Roger's "Mountain Poncho" but to me it is functionally closer to a parka than a poncho.