For me, from purely an 'appearance' perspective, definitely the GT3. The relatively minor GT3RS changes are mainly cosmetic. Are they really worth an extra Pounds20000? Not to me!
And, from an 'overall' perspective, again the GT3. Same power and performance for much less money. Plus the GT3 allows a PCM and other creature comforts so it's the best of both worlds within the hardcore, track tool segment.
That might sound funny to some here, but if you're driving from the UK to somewhere in Germany for a track day, having a PCM with satellite navigation is really useful.
Just a little example, I was near Winchester last Sunday meeting Porsche-Jeck. I only knew one way back to London using the M3, M25, M4 motorways. They were horribly clogged up with traffic and roadworks. Literally, the M25 was like a car park! But the satnav 'avoided motorways' and took me on a 'dynamic route' using data from the Traffic Messaging Channel so I didn't have ANY traffic. It took me along 'A' roads that I had never even heard of which were empty. I was literally laughing inside at how great a job it was doing! That 80 mile journey took me 1.5 hours instead of about 3-4 hours. Money well spent. If I bought a GT3, I would still want such useful technology. (Although I fully appreciate that some here would not e.g. to save on weight).
Just my 2 cents