The 968 has more creature comforts, it's a newer car. It would also probably have lower miles. IF you find a good one a couple of states away it wouldn't be a bad idea to fly there and drive it home. Good examples of these cars are worth it. I personally love the look the 968 coupe. I don't like the cabs so much. I would also venture to guess that the 968 would be a tad bit more reliable since it is N/A. Where the 951 lacks in creature comforts it makes up in capability. It is MUCH more capable of being souped up. Just look at some of the monsters on this board. The 951's would prolly be a little cheaper to maintain, but would require more maintainence. Either way, both have decent stock power, and a fair amount of creature comforts, but ultimately if you want a really comfortable car a Porsche is not for you.
Good luck looking!