From a novice point of view..
I wouldn't recommend any car as an investment - have a look over recent posts here - people have been discussing such matters at length.
The 348 was not much loved, under powered, and not developed all that completely - a lot of people felt it was an unfinished car. It's not the prettiest of Ferraris either. If anything like other Ferraris it will need more frequent servicing, which wont be cheap. More consumables which are likely more expensive, it will be far less reliable and not a practical daily drive. A lot of Ferraris are quite low mileage - there is a reason for this, they are not great every day cars. It will devalue more if you use it a lot.
Which begs the question - what are you going to use the car for?
As for the 996 Turbo (which is the 911 turbo that falls into your price bracket) - much much much faster, much more reliable and useable, better built. Its a highly respected Porsche (vs a less respected Ferrari). Better service intervals I think and make excellent daily drivers.
The combination with the Tiptronic is not the favourite one, but it is nigh on indestructible as far as the amazing Metzger engine goes and the Merc gearbox. Bullet proof.
Both cars will cost, they are likely getting long in the tooth, keep some cash in reserve for a rainy day.
Cost up a new set of tyres, road tax, brakes and a service. You may be surprised. I don't drive a turbo, but tyres and break (PADS) only cost me about £1100 iirc, add into that discs & tax and its getting more expensive.
Good luck - buy the Porsche, it will reward you more day after day, with less heartbreak and a lot more speed and safety.
M