Yeah, that's a problem - I think its called pre-buyer's remorse.
But I'm inclined to agree with Bill, you should at least have a chat with your seller to honor the agreement.
And the '68 is probably a better deal in the short run. Since the 1978 has only 850 hours in 30 years, where has it been? Has it flown recently? Planes just don't like to sit, even in a hangar. They were born to fly and stay in their best condition when they do.
