To tackle your second question first, I'd agree with Pyrex that the SR entry for dimensional lock is possibly erroneous. The descriptive text certainly seems to contradict it and I believe the text is supposed to have priority. In the case of forbiddance the spell description states that "A successful Will save halves the damage, and spell resistance applies", so presumably SR only applies versus the damage and not the blocking of extradimensional travel.
As for differences between dimensional lock and forbiddance I think there are some important distinctions. Here's my current understanding of what they are.
Dimensional lock states that extradimensional travel is blocked "into or out of the area". Forbiddance states that extradimensional travel is blocked "into or within" the area. Both spells make it impossible to translate into the warded area from another dimension. Dimensional lock also makes it impossible to translate out of the warded area to another dimension. The specification of "within" in the case of forbiddance is a clarification of "into". While casting dimension door within a dimensional lock fails automatically (translation to the Astral plane is blocked), it is possible to cast dimension door within a forbiddance (translation to the Astral plane is not blocked) but if your destination area is also within the forbiddance (dimension dooring between points both within an area protected by forbiddance, for example) it will fail (you cannot translate back from the Astral).
Furthermore, dimensional lock states that it "does not interfere with the movement of creatures already in ethereal or astral form when the spell is cast". I think this is a poorly worded part of the spell but what I think it means is that creatures in ethereal or astral form are not blocked from passing through areas of the Ethereal or Astral planes that are conterminous with a dimensional locked area. They cannot translate while in such conterminous areas but are not impeded upon the Ethereal or Astral planes by the dimensional lock.
Forbiddance, however, states that it "seals an area against all planar travel" with no exceptions. Creatures in ethereal or astral form cannot cross into areas of the Ethereal or Astral planes that are conterminous with an area of forbiddance. So, while an ethereal spy could enter areas of the Ethereal plane that correspond with the area of a dimensional lock to see what is happening in the immediate area of the Material plane, they could not enter an area of the Ethereal plane that corresponds with an area protected by forbiddance thus making such ethereal spying impossible.
To sum up, forbiddance permanently protects a large area from all forms of planar intrusions as well as some non-planar intrusions while also not trapping you within it from escaping rapidly if necessary via some planar means, while dimensional lock temporarily (although the duration is pretty generous) protects a small area from planar arrivals or departures. Forbiddance takes a while to cast and costs a fair amount in material resources, while dimensional lock is quick enough to be used in a combat situation and is also cheap.
