The GR sights on the Supernova are of high quality.
In terms of the Remington 870P...I too desire one of these. However, from what I gather, they are not perfect and according to some, may be a bit overrated. Still, it seems like a good shotgun.
Anyway, the Benelli SN is a solid choice; One of the best buys for the money (initially speaking), IMHO.
As a side note- It is unfortunate you had such an experience with Mossberg. Generally speaking, they are usually fine weapons.