Sorry about the delay. Choosing between GS-150 and GS-125 is more of a personal choice.
If one wants a 150cc bike GS-150 is the only choice. If one wants a 125 cc bike then there are several choices (other brands). If one wants a "Suzuki" and one of the 'heavier' models, then one has to decide between GS-125 and GS-150.
Obviously, GS-150 is more powerful, as both the bikes weigh about the same, the performance will differ according to the power of the engine.
Other than power the main differences are cost related.
150 is more expensive than 125. (one time expense)
150 is more expensive to ride (fuel consumption. (regular running cost)
Both cost about the same to maintain as almost all body parts and many(most?) engine parts are the same.
I am not aware of the price difference between the two or the fuel cost difference. If someone can afford to buy and maintain, then GS-150, but if one has some budget constraints then GS-125. Also, sometimes there are some issues that are seemingly unrelated to us, but are important for the buyer, and they have to go for the lower or standard size/budget.