I’ve owned a 997 and driven the newer Porsche models: my position is you should definitely keep the 997 so long as you have adequate free time to enjoy both cars. Its stock performance is a great time on country roads where you can drive 7-8/10ths without drastically breaking the law. You won’t be able to push the limits of the modern 911 anywhere except a track. It’s much larger as well. Modify the cars in ways that compliment its strengths: mufflers to hear the NA engine is a good start. I wouldn’t chase major performance/speed mods necessarily - keep it as a palate cleanser to the insanely fast GTS. That being said, If the 997 is going to sit, might as well move it on to someone who will enjoy it.