I was always told that the solid spacer helps cut down on them getting noisey over time, if he doesn't like them that's cool too since most install kits I have been looking at have the crush sleeve anyhow...

 

I cant speak for Harold but the 4.10's i did in my original 10 bolt never got any louder after break in and i used a crush sleeve. My 4.10s did eventually break but not because of strength or set up, its a long story but basically richmond recommended some grinding on 4 of the ring gear teeth to clear my posi unit and those 4 teeth eventually broke off...

The only reason i dont like the solid spacer is because it is very hard to get an acceptable bearing preload, add a shim and its way to tight take one out and its not enough...thats how harolds was when i was setting his up. If we end up doin your rear we could always try it and if its being a b!tch like harolds was there is always the crush sleeve...I never heard the opinion that the solid keeps them quieter longer, most say it just adds some strength.