PDM - Well, the top down is kind of a big thing, but if your'e not a ragtop guy that's not necessarily a biggie. I have had a convertible "sports" car since I bought my first 1970 corvette convertible new, so to me it is quite a big thing. Another big difference is the feel of the car, the Porsche has a feeling that can be described as refined, but I also felt mine had a bit of "fragile" in there along with the refinement. I had a 1974 Porsche as well & it didn't give me the fragile ******* like the 2000 did. There are a lot of systems on the current Porsches and if you check the Porsche boards you will find a lot of complaints about many of them. The GT-3 will have some features my Cab did not, including a more fully adjustable suspension that will allow you to dial out the standard understeer, but my Cab would hav required aftermarket adjustable strut tops that would not be allowed in scca stock classes. My Porsche paint chipped very easily, but that could have been my particular example. The SRT is heavy, tough and indestructible "feeling" and I prefer that feeling overall. The biggest difference is the immediate torque of the SRT compared to a lower torque higher winding engine of the GT-3. You will be noticed by fewer people in the GT-3, particularly if you go with a subtle color like silver, but I might actually prefer that aspect.
Remember, I didn't say the GT-3 was a bad automobile or that I wouldn't consider owning one, just that it was a very different experience than the SRT. I am not a big drag racing fan, so the few times our local VCA goes to the track - Rte 66 and rte 41 locally, will be more than enough for me. And as our Region has decided to allow the SRT to run Gingerman with only Harnesses as mentioned above, I should be able to play enough for my tastes. I liked the 00 Porsche Cabriolet, but I currently have a BMW E46 chassis car and find it to feel just as refined as the Porsche, but with less of the "fragile" feel I took from the Porsche. You might want to look into the new M3 lightweight coming out soon, It should be quite fast, but again, a totally different experience than the SRT.
Either way - Good luck with whatever car you end up with.