I'm a bit late to this thread, but I figure I'll put my two cents in just for good measure.
The switch from C4S to California, imo, is a far more different in price than in car. The cars are no doubt drastically different, but the price even more so. I think it's an odd jump with that respect, but if you can afford it who cares.
Maybe I misread, but was having a convertible mandatory? Was the C4S a cabrio? I know you've ruled out the R8 already, but have you ruled out the R8 Spyder. I love the R8, but think the Spyder is hideous. A lot of others think it's awesome. I've heard a lot of killer reviews about it. In none of the available forms of the R8 is it the fastest car around, but it would run circles around the California in either V10 form, and like I said, the Spyder has been getting much praise.
I'm not sure that I'd say the California is any more practical than the other two seat exotics. Yeah, you get a trunk and a bit of bench room behind the seats, but it's not like the thing is a sedan. If the most you'll ever take is a week long trip, you could easily fit the luggage and such into a Gallardo, R8, or anything with a small shelf behind the seats.
I'd also hesitate with the California due to price. It's mad expensive (it's a Ferrari) and will see a lot of depreciation with not too many miles (especially if it's new!) There are a lot of very good cars out there with a few thousand miles on them with far lower prices. If you're willing to pay MSRP dollars for a new Cali, you could probably find a used 599. You could also get a Gallardo Spyder, LP560/Spyder, R8 V10/Spyder, 599 with a few miles, and the list goes on. You could walk away with many great cars for a lot less money, hell, you'd probably have enough left over to buy most of a 991 too. I even see a 2010 560 Spyder on eBay right now for $199K. Not my favorite colors, but who knows.
Having said that, if it's mandatory to have a convertible then your choices are more limited, but for what you're paying, there are sooooo many choices that I feel best a California in a lot of way -- dare I even say a Bentley SuperSports? And also, so many modern supercars can be DD's and GT's. All the recent cars are not only supercars, but GT's -- albeit with smaller luggage space. Seriously.
I'll give in and say the California is a bit unique and it's grown on me. I even saw a red one a while back as it drove by and thought the rear end didn't look too bad afterall. Still, I don't think it's the best choice, especially if buying new.
