You will of course find people on this forum who own all of them. Well, no one person is likely to own all of them but all of them are owned by somebody. Well, all are owned by different people. You know what I mean. Anyway, that means there are adherents to each of the models, people who are quite happy with their choices. That being said...The more I've researched the more I've moved from the GT4, to the GTS, to the S, and now strongly considering base with 18s.
Still PDK, PVT, & PASM though!
Each model has its strengths, but none are deficient in ways that matter for most uses. All have plenty of power. All have great brakes. The subtle performance advantage of the S is nice but it comes at a cost. The GTS and GT4 even more so. You have to be realistic about how you'd use it and what is important to you.
The wheel size is another matter people disagree on, but in fact as far as I can tell the differences are subtle. My 20" are nicely decorative and their low profile tires probably give it more tracking precision. I've got 19" wheels with winter tires on it now. They look more conventional, and I've been told they make the car look faster. I dunno' about that. They feel more conventional, probably because of the compound as much as because of the profile. The ride quality is marginally softer with the 19" winter tire setup but that's probably because of their lower pressure. The 20" wheels have one minor disavantage, they are so low to the ground that they can scrap against curbs. Other than that, I'm not sure I could tell the difference. In a rational, cognitive way I like the 20's as more extreme. Would it matter if they were 19 or 18? Nah.
There are other considerations more important. Like manual vs. PDK, PDLS or PDLS+ or standard lights, multi-function steering wheel, audio system, environmental controls, PCM functionality, etc. As long as you meet your minimul requirements for those things you will be happy with anything.