These questions involve complex issues, some of which have been covered.
Napa is a very good region for Cabernet Sauvignon and some of it is terroir, although if you drive up Highway 29 through Yountville and St Helena, you'll find high end, highly rated cab producers right next door to the wine world's "mid-fi" producers.
You might also be surprised to find that a visit to Bordeaux will reveal that wines with different flavor profiles like Margaux and Latour are situated right next door to each other.
It also depends on the style of wine you like to drink.
You also have to factor in the fact that wine can vary quite a bit from vintage to vintage.
And wine making styles have also changed over the past few decades.
There is also a difference between wine made from grapes grown on the "floor" of Napa Valley versus Cab grown in the mountains above.
So, to be really specific, you have to ask which vintage and which producer of Napa Cab versus which vintage and which producer of French Bordeaux?
To make very general observations, Napa Cab, coming from a warmer climate, tends to produce riper fruit, but then the best vintages of Bordeaux have also been characterized by riper fruit -- part of what makes them better vintages -- better weather. Then again, wine makers have been getting more and more scientific and have learned how to trim away leaves to expose the fruit to help ripen grapes in less favorable vintages and the trend throughout the world in the past 20 years has been to pick later and later, waiting for fruit to get more fully (and some say over) ripe.
Here's another wrinkle;
Back in 1976, there was a wine competition held in France where Napa Cab was judged blind against French Bordeaux.
The judges were luminaries of the French Wine establishment, the very type of judge who would rather lose a limb than see a California wine surpass a French wine in the competition.
Guess what?
A California Cab won.
There was also a competition between California Chardonnay and white Burgundy if you were to guess that a California Chardonnay topped the ranking, you'd be right.
What we do know is that Bordeaux has a much longer history, so we know that wines produced there have established a history of aging and improving over lengthy periods.
People are still drinking these wines and some are still waiting for wines to peak that were produced 50 to 75 years ago and sometimes even longer.
The generalization is that Bordeaux will age much longer than California Cabernet and in most cases that would be correct, but some vintages of some california Cab will age longer than some vintages of some Bordeaux. If you want to buy a wine that you can stick in your cellar for 20-30-40 years and you'd like to taste the type of wine that can result, in general, you'd be better off buying Bordeaux.
If you want a warmer, bigger, fruitier, earlier drinking wine, in general, you'd favor Napa Cabernet, but even there, the best examples should really be aged at least 5 to 10 years before drinking. IMO, most people do not get to taste California Cab at its best because most people drink it too young. Most are exposed to it in restaurants and very few restaurants carry older vintages because few have the room to cellar a lot of wine for lengthy period, so you generally see very recent vintages.
If you want to get an idea of what an excellent Napa Cab can taste like with age, seek out a 1991 Chateau Montelena or 1992 Beringer Private Reserve.
These Napa Cabs are in their prime right now, will show you what California Cab can do, will provide a fabulous drinking experience and will cost you a fraction of the price of an aged Bordeaux.
Have them with a nice simply prepared Steak and your amplifier will go to 11.
