Your post is confusing. The title says trading "348 for 997" but your post says you are keeping your 348 in NYC but trading up - what vehicle are you trading for the 997?
The 348 is a raw, hand built, Ferrari, the last without power steering and all the other modern technology. I have driven many 997's of almost all stripes. They are modern sports cars and the distance between a modern Porsche, BMW, and AMG etc, in terms of driving enjoyment can be measured in inches. An older Ferrari or Porsche is a much more involving and entertaining experience to me. I would find your consideration more compelling if you were looking at trading for a Porsche RS America or 964, heck even a 993. 997's are fun, reliable cars that can be driven daily. A 348 is an event. Just my .02
I considered and drove several 997s before getting the 348, all S models. TOTALLY different experience, so I guess it all comes down to what you require or what gets you goin' at this time. I would have one, but not at the expense of my 348.
I think you are asking this question in the 348 section of a Ferrari message board so that we will talk you out of it.
I've been fortunate to drive a few different 997's also. And I've driven a few 348's. Honestly, the 997 is quicker, quieter, cheaper to run (and probably insure), has much better economy, roadholding (they grip like nothing else) and is (lets be honest) a far better car than the 348 in every single respect. Except the one that matters most - enjoyment. As a daily proposition, its superb. As an occasion, its a non-event. Up to you to decide what you want from the car.
A 348 is a Fiat Coupe if the 997 is a VW. In this case, it all comes down to what you're looking for. A 997 is an immensely capable car, for cruising and having fun in it's a wonderful thing. It looks good, it's well made and is likely to start every time and not need too much fussing to just run it. The 348 being older and Italian is more likely to be temperamental, though I'm assuming if you look after it then it's as reliable as anything else on the road. It's just whether you want to keep looking after it to the extent it remains reliable, something which would probably be less hard work for a newer Porsche.