The P-97 is an improvement/refinement of the P-90, with an improved barrel/lockup design.
The older P-90s were not known for their accuracy, but were highly regarded for their durability and ability to feed ANY ammo. The P-97 is more accurate, just as reliable, and maintains the Ruger tradition of feeding anything of the right caliber.
Nice gun, and a great place to start if you don't already have a .45.