There should really be no sonic difference between placing an effect on a track versus a bus. There are advantages and disadvantages to both methods, but they don't have to do with how the signal is processed within the effect, only about how the signal is routed in the project.
Putting the amp sim on a bus allows you to send multiple guitar tracks to a single instance of the plugin, assuring a consistent tone across multiple takes and minimizing CPU usage (versus many instances of the plugin). It also accommodates parallel effects and easy wet/dry adjustments and automation.
The downside to bus effects is that you can't freeze busses, so if CPU usage is getting out of hand you'll have to bounce to a new track rather than enjoying the convenience of the freeze function.
Another big limitation of bussed amp sims in particular is that most of them are mono effects and therefore impose mono-ization of a stereo bus. For example, you can't have two guitar tracks panned 100% apart and use sends to route them both to a common amp sim on a bus without losing their pan positions.
Personally, I usually put amp sims in as track inserts. Amp sims tend to be high-latency CPU-munchers, so I typically freeze them once I'm happy with the tone.