Right so I'm a sussex 3rd year who lived in northfield, but has many friends who have lived in stanmer park.
Firstly stanmer park, yes is right next to the train station, but that means its closer to the bus stop (which is literally just opposite the train station - you turn left when you leave your accommodation to get to the bus stop which is right there, and turn right is the train station) so in terms of commuting into central brighton or getting about is ideal - but the noise i can't speak for because I didn't live there. Although I have been in stanmer park, and theres very few windows, which to me made it feel a but like a prison more than anything else. Ontop, all the med students live there (which are the reason I know so much about it) but frankly, med students are annoying as hell to live with / near - because as much as they say they are accepting, they stick to their own, and if you aren't a medic its hell trying to break into that crowd.
In terms of northfield, I can see what you mean when you say its out the way, but this SHOULD NOT BE OVERSTATED. At MAX its a 10 min walk from what you would call the main campus which is library square, (probably could do it in 5/6 mins if you dont dawdle like I do). Regardless northfield is a nice option, its newer and pricier, and on the downs so instead of the train station you get fields and sheep and lovely views etc.
I'll cut the waffle, as a current student here who has seen both of these accommodations, definitely pick northfield. When people say its out the way, its not. If any accommodation is out the way, its stanmer because its technically not on the campus its just outside (literally right outside) but when you're here you'll see stanmer is way more removed from campus life than northfield.
Basically, i've given points about both, but go for northfield. Its better in every way.