Both are fast acting, but because the injectable form does not filter through the liver on the first round, it is faster, if slightly, than the oral form.
Taken orals roughly 45 minutes before a work out puts the drug at Full effect. The injectable is usually bio-available immediately to within a half hour for full metabolic use.
Liver Toxicity is an issue with both the oral and the injectable forms of these. The injectable is slightly less toxic than the oral formula. Overall the only real benefit of the injectable is lower toxicity.
The results on bodybuilding are the same regardless of which form of the drug you use. The biggest downfall of injectable is obviously the concern over the quality of the injectable.