Flying into Manchester from the US instead of Birmingham

I am contemplating booking flight tickets to Manchester instead of Birmingham and taking the train from there, I am looking at about $650 savings for very similar flights from the US + I would have to add the train ride of course from Manchester to Birmingham.
Any good reasons not to do that? Are trains generally reliable, are the advertised train timetables accurate? There's no heavy luggage to haul around at most a small carry on.