The difference is that often sales commissions have no (or much higher) upper limit. That is, if you are skilled enough (and lucky enough, for sure, since there are certainly many factors outside of the salespersons control), then they can potentially make a large commission. On the other hand, a salaried engineer has a hard upper limit (their salary + any bonuses that may exist) which can only be raised through a promotion or renegotiation.

A problem for me is that there's little incentive to work b tons what my contract stipulates and what my manager feels is acceptable productivity (and whatever I need to get bonuses, if I get bonuses at all), because I get the same compensation (in terms of money or time off or whatever -- I don't get to leave early if I finish my work early: I move on to the next task instead) no matter how hard I work or productive I am. So why would I wear myself down by working harder if I can be less stressed and work less hard for the exact same compensation?

At least a commission-based role puts some of this into my hands. If I feel I can make more sales by working harder/longer/faster/smarter, I can get paid more.