8 speed drivetrain stronger than 9 speed?
My guess is in reality no statistically measurable difference between 8,9 and even 10 speed setups. 8 Speed these days is low cost components, meaning you are fitting out a bike with low cost gear. Even 9 speed is lowish cost. 10 is geting into quite fine tolerances but has been around long enough to be well understood. 11 and 12 speed setups might be a concern, but I would not fit them because of concern over strength. 

More important though is "Strong enough" - once the gear is "strong enough", does it matter if one is stronger than the other. And setup built with components of good enough quality to be used where reliability is a concern is strong enough.

The disadvantages of more gears is finer tolerances meaning wear is a bigger issue. Wear is a different concern to strength as it affects everything but is predictable, and easily managed with normal maintenance. (Cost is another discussion)   

There might be reasons for not going to higher gears - availability of components in remote places in the event of a break down. However going 9 speed over 8 speed has little disadvantage here. Most components are interchangeable between these, and if needed you would as easily get you bike up and running on the off the shelf components at the most basic bike shop  with 9 speed as 8 speed.

As eluded to earlier, cost might be a reason to stay on 8 speed - but that is a different question (and I have my doubts it is significant).