The whole "there will always be something newer coming soon" argument is valid, but there are cases when I say it holds to less of a degree. An example of when it does not apply is when a phone is coming out with a cortex A9 processor more than a year after A9s were introduced on phones. Yes, clock speeds have been steadily rising that whole time, and benchmarks and performance has been improving, but these have been essentially modifications to the same base. A15 will be a new base and be a leap forward vs. little steps forward. That's why it makes sense when a A9 phone comes out (way after A9s were first released) to want to wait until the A15s hit, so that you can upgrade at a time when a great leap is made vs. upgrading at a time when only little steps are being made.
Now the GS3 with the S4 is kind of close to a leap, since the S4 is more advanced than cortex A9, but the thing that keeps it still just a little step is the GPU. Adreno 225 (and even the international's Mali) are both undeniably little steps forward vs. leaps. They are effectively mild improvements over 2011 GPUs, vs. Adreno 320 and the upcoming Mali which will be a new gen. The way I see it, I would MUCH rather make the decision to upgrade when a whole new generation of specs comes out than make the decision to upgrade at the end of a specs cycle. For those into cars, here's my analogy: the GS3 (and One X) are kind of like the last model year of a midcycle facelift. Compared to the initial release of the generation, the styling has been mildly updated, the HP and torque have been bumped (but they're still using the same fundamental engine, just tweaked for a bit more performance), some nice new features have been added to the interior, and things like the suspension and brakes have also been tweaked for performance. Still, it would be a bummer to get the last model year just before the next generation comes out, which is more than just an update.
Ultimately, it depends on where you are coming from, since if you're coming from a really old phone then the GS3 or One X may be a completely new gen to you. But if you're coming from a 2011 phone, you're essentially looking at the last evolution of your generation, which is why some people might want to wait until this fall/winter to see specs that so eclipse their phone as to warrant their designation as a completely new gen. That's where I'm at. Yes, the One X and GS3 are both better than my Sensation, but not "enough" for me. That's of course subjective, and I know the upgrade addicts probably don't agree, but there are also a lot of people like me out there.