The price is the real issue. The getrag is very expensive, even used. The reason they use a gm tranny is an automatic thing. The r154 is a good tranny for a reasonable price. Its not near as strong as the getrag. Stock getrags are known to hold 700 to 800 wheel hp. And yes they do break depending on how hard they are abused. Just my .02. The r154 have thier own issues as well. Both are good trannies.
All depends on budget and intended use for what you want.