A few thoughts here:

1) price - both are what I would consider pretty good prices. According to pinside, Corvettes average 3650 and bad cats are at 3k. In truth, that's about right for corvette...but pretty low for bad cats.

2) availability - they made 5,001 Corvettes, but only 2,500 bad cats. Bad cats is a very quirky theme and pretty desirable since it is a system 11 and "cats and pinballs" are inextricably linked. People just don't sell bad cats pins much. They are FAR harder to come by.

3) fun - Corvette is newer with more modes, more shots, and a good/different layout. Only you can answer which is truly more fun for you, but most would likely say corvette (the top 100 has already spoken: Corvette is 125, Bad Cats is 158)

4) X-factor - these themes couldn't be more different. I would go with bad cats, but everyone will be different. This comes down to personal taste.
I don't recommend buying pins based on what other family members want, especially kids. I have 3 kiddos, and they are usually interested...for about 2 seconds. They are just happy to play, but they are fickle. You are spending the money, making the deal, and likely playing it the most by far. Get what YOU want.

You could always buy one and sell it down the road to acquire the other later.

Bottom line: Corvette is the better game, especially for longevity...but bad cats is much rarer, harder to come by, and probably the better priced of the two (it will certainly be easier to sell down the road). I feel there are better games for the price of either, however. Are you locked into these two? I would browse your options if you have time .