When I bought my 993 the difference was a little more than that. Styling aside , the question is like is the premium between an SC and a Carrera or a 2.7 mid year and an SC worth it.
Each generation is better than the last in some ways and the same in other ways.
"Vive la differance"

I think the 993 will always be more valuable than the 964. There will also always be a premium for lower mileage cars - particularly the C4S widebody coupe and Turbo. Those cars will appreciate. There will be others that struggle for value - like a car with a bad color choices and a tiptronic. Regular coupes and cabs will be solid, but limited in appreciation - mileage and rarity will drive the cars that appreciate.

There will be a floor for a reasonably well maintained car regardless of the mileage - somewhere between $20K and $25k. There are more and more 100k+ mile cars out there that run well, are well maintained and have good records.

My car isn't neat and tidy (if I have enough time to be detailing the car, I'd rather be finding a windy road somewhere for a drive) and I still get comments periodically on the last of the air-cooled cars or what a great shape. I have 82k+ miles on mine and intend to drive the snot out of it for many years to come. I love the sound and there is absolutely no substitute for the sensation of the car getting planted in a corner and hurling you out the other side.

Tips are definitely a negative.

The long term value of cab vs coupe is a well worn topic, with general (but not universal) agreement that pre-993 targas will always lag (except the soft windows).

The 993 targa is a different question, since it is more rare, and more coupe-like. Any thoughts on those? I haven't seen a big enough sample to make a guess.

'Targas don't never get no respect', and 993 Targas probably get less than that. While someone here will probably say his has been a perfect car, the tops can be VERY problematic.

FWIW, when I decided to 'upgrade' my 356 Roadster to a more modern car, the Targa was the only one I looked for. It is quiet & waterproof (really), or top down, depending on the day. As far as I'm concerned, it is the one car to have if you're having one.

I was actively looking for a SC Targa when I bought my Cab. Mainly for budget reasons. "Less desirable" by the broader market is not a bad thing when you are looking to buy a less expensive car!

But I stumbled on a deal I couldn't refuse.

I could see where a 993 targa could be the worst of both worlds: squeaky, balky, and leaky, without giving the same open air motoring experience. But I've never even been in one, so I don't know if that is true.

An early '95 built in 94 has a 2nd gear the slides off.