What a crock! Said it before and I'll say it again... when driving in a spirited fashion an R154 is likely to break before a W58.
In theory the R154 is stronger but the thrust washer will mince the thing in no time! Mince a piece of metal through the gears and even the strongest box in the world would be toast.
I'd suggest that you modify the R154 - there are two upgrades that are particularly important. Firstly replace the thrust washer.. and secondly if you're going to do any drag racing or looking for 500-600hp you should consider a billet input shaft also.