P-89 is a steel framed gun, and the P-95 is a similarly styled polymer framed gun.
They perform pretty similar from what I've heard. The P-89 can take aftermarket grips but the P-95's grips are molded in and fixed.
I personally have the P-95 and it's a good gun. Feeds just about anything you wanna shoot. It was my first handgun that I bought though and I wasn't really familiar with lots of differences (I settled on the Ruger because the price was attractive and it was well recommended) - these days if I had the choice I'd personally go with the P-89 as I've come to prefer a metal frame over polymer. Either will do you just fine though.
PS Not to try and sway you, but there's some REALLY good deals out there on the SIG P6's right now (imported police trade in's from Germany). As I said I've got a Ruger P-95 as well as a SIG P6 and while there's nothing really wrong with the Ruger, if I had to keep just one it'd be the SIG.