If you go to the train station, and up to the top level, you'll have an EXCELLENT view of both.
Be aware though - even in the off season, those spots can go more than an hour in advance of Spectromagic.
What I personally like to do is watch Spectro from somewhere in the middle of Frontierland. Then, once the last float has passed, use one of those breezeways to cross into adventureland, head over to the crystal palace and then through the shops on Main Street until you're somewhere in the middle of main street. Then, just make sure there's no roof over your head (ie those little porch coverings). The further back, the better, but you'll have a nice view no matter where you are on main street.