I feel the need to put my oar in here becuase off all the bullshit I hear about these gearboxes, some real world info that may help people out somehow:
The old chestnut is "W58's are weak and shit, if you driffxors with any more than 300horsepowers they 'splode"
But as anybody with a concept of brick walls knows, horsepower doesn't break gearboxes so much as torque, so, a very general consensus of supposed torque handling figures before regular breakage:
Although its all a mute point, do you drive like brian johnson or boris johnson? is your car running space savers or 19" slicks? do you have a 4 puck triple plate clutch or a clutch bag made from duck?
Anyway, you get the idea. I kicked the crap out of my W58 on an almost daily basis for 2 years, it was only behind a N/A 2JZ-GE but was missing its spigot bearing (dont ask) and its still ok now, bit of a whine but drives perfect (or it would if it wasn't festering in my back yard)
R154 is a truck gearbox (or variant of, they used the same design in some modelsof hilux and jeep cherokee, i shit you not) It shifts like a truck gearbox, especially compared to a W58, but is a good bit of kit, not unbreakable but pretty damn strong, but then both boxes are compared to the 200ftLb wonder fitted to S-bodies and the R32 GTS-t, and the similar-strength ford Type 9 and MT75 to name another couple of things I'm familiar with!
What I consider to be a bigger problem with the W58 vs. R154 dilemma is the clutch. In most applications that we will be involved with (not trucks and jeeps) the R154 uses a pull type clutch, which generally speaking have more clamping force than the conventional push type, which the W58 uses, but with a worryingly small friction area, so it doesn't usually last that long even with standard power, although upgrades are available and if you are going the 1UZ route with either box the clutch setup will most probably be push type unless you can be bothered with a lot of fab work for no real gain over a decent upgraded aftermarket push setup.
So after all that I guess my point is bear all this in mind when you are choosing!