Kelty cosmic down 20 or 40?

I have a Kelty 40 degree bag and wish I had a 20. I'm in New England which is pretty similar latitude wise. You won't need it for your July trip but you will have a lot more flexibility with the 20 degree one. You should be able to vent the bag enough not to overheat.
That being said, I am going to be switching to a quilt (probably from Enlightened Equipment) so I can vent it better for use in warmer temps.

Thanks for the advice, I think I'll Def go with the 20. Any idea on sizing? How does yours foot you? Just wondering if I should go regular or long. I'm 5 10 and 195

To offer an opposing perspective, I had the 20 and found myself overheating easily in it. The 3/4 length zip made it impossible to vent my feet without sticking an entire leg out. I too have now switched to a quilt for the lower weight and increased ventilation options.

I got my Kelty used for a very good price, so it was a good purchase as a first down bag. If you're looking to buy new though, I strongly recommend saving up a little more to get something that's lighter and more versatile. I went with a Burrow 20 from Hammock Gear, which is less than $100 more than a new Cosmic Down 20. Buy it right the first time and you'll only cry once.