if you are geting a 9" then get 31 spline axles. if you get 28 spline axles then your wasting the strength of 9". the 8" has 28 spline axles also. the only significant strength increase is that the cap bolts are bigger and the teeth are bigger. axles are usually the first things that gets broken in high power cars.
Had a Mustang II with a pretty healthy 351W, and a 175 shot of nitrous. 8 inch rear w/4:56 on a spool, stock axles on a 26x8 slick, she'd run 6.40's all day. The 9inch is definately better but, I'm just sayin . . .
The first things to go in an eight-incher, with any significant amount of torque, coupled with a sticky tire, are the caps or pinion support. I have seen more than one eight-inch do this in 12-13 second cars. If you have never seen this happen, it's kind of like when a mean dog takes off after you, then suddenly gets to the end of his chain! I'm sure for every "broken eight-inch" story, there are many more "my car has a zillion hp and my eight-inch was fine" testimonies. You just have to decide if you want to be the one-of-ten, or one-of-twenty that experience this.
Ditto, no point in going with a 28 splined 9" unless a 28 spline 9" center section falls into your lap for free. 31 splined trac-loc 9's were a little more numerous if you're going J'yard hunting to look for one. 4X4 Pickups and Bronco's.
It pretty much depends on how you will use it. If you plan to mash the throttle or "dump the clutch" then you will have to replace the eight inch anyway - and probably a driveline and u-joints too. If it is a Friday night cruiser that you don't plan on racing (or showing off) then the eight inch can last a long time. The key is smooth acceleration instead of drag race acceleration.
