I've done CT to Ann Arbor via Canada and via Penn/Ohio, they're really close in time (I forget which was faster). Driving through Pennsylvania is a drag though, such a long state....and the radio stations on I-80 don't usually last for more than a song or two (not that they're any good anyway)

Going to Detroit is faster via Canada just because Detroit is essentially bordering Canada. Though I'm not sure how much going from NH makes a difference....maybe half an hour? You still have to take I-90 and from where I live in CT to where I get onto I-90 is a pretty short distance.

EDIT: according to Google Maps, Manchester to Ann Arbor is 12:52 via Kings Highway (Canada), 13:24 via NY/Ohio (I-90), 13:51 via Penn/Ohio (I-80, which probably will be under construction anyway). So only 30min extra to stay in the USA, not bad No dealing with the border either, which is always a plus in my book